J. CHEM. SOC. FARADAY TRANS., 1993, 89(11), 1623-1630 1623

266 nm Photolysis of HOCl and DOC1 : Laser-excitation Fluorescence Detection of OH and OD

Christopher G. Hickman, Nebil Shaw, Michael J. Crawford, Andrew J. Bell and Jeremy G. Frey* Department of Chemistry, University of Southampton, Southampton, UK SO9 5NH

Fragment rotational population distributions for OH and OD were observed by laser-excitation fluorescence following t h e 266 nm photolysis in a free jet molecular beam of HOCl and DOCI, respectively. The photo- dissociation dynamics show a strong preference for the 211(A') OH or OD lambda doublet component in both spin-orbit states. The 211312 state is more populated t h a n t h e 211,12 state of OH. The distributions are consistent with photolysis via a repulsive excited state and the measured rotational alignment (-0.28) is compatible with an excited-state symmetry of A' for t h e dominant dissociation pathway. The h ighJ limiting value for the degree of electron alignment reaches the extreme value of - 1 for the 211,12 state for both OH and OD, but falls short of th i s for t h e 2113,2 state, reaching only -0.7.

HOCl has been implicated in ozone depletion in the strato- sphere as a possible Cl and OH reservoir, and in the Antarc- tic stratosphere because of the reactions at ice surfaces that generate Cl,. In both cases the photochemical stability of HOCl is important in assessing the atmospheric lifetime of HOCl and thus its actual importance.

There has been considerable debate on the nature of the HOCl electronic absorption spectrum especially at wave- lengths longer than 300 nm. HOCl has a strong absorption at C Q . 250 nm and some experiments indicate a second maximum at ca. 320 nm.' Ab initio calculations were unable to reproduce this second and this led to sugges- tions that the second peak was due to other species (particularly C1,O) inevitably present in the HOCl samples. Calculations on the HOCl.H,O cluster5 showed that the absorption spectrum should be similar to HOCl and thus clusters could not resolve the difference between experiment and theory.

Two recent very high-level ab initio calculation^,^*^ using the MIDI4** basis set (split-valence with polarization), while predicting a similar pattern of HOCl excited states to the pre- vious calculations, predicted the excited electronic states to be at significantly lower energy than the earlier work. Tran- sitions were predicted at ca. 320 nm (to an A" state) and ca. 250 nm (to an A' state). This is in close agreement with the UV spectra observed by Molina and Molina' and Mishalanie et aL9

The previous experimental work".' on the photolysis of HOCl and DOCl at 248 nm indicates that the dissociation proceeds through a repulsive A' state, in agreement with all the theoretical calculations. The excitation would have been on the tail of the strong A' absorption (the excitation to the A" state is expected to be much weaker). At 266 nm the earlier ab initio calculations indicate that the excitation should be much closer to the maximum of the A" state. The most recent calculations would predict that at 266 nm the excitation should still be essentially entirely due to the A' state. Since the rotational alignment of the OH (or OD) product depends on the symmetry of the molecular transition moment, photodissociation experiments at 266 nm with laser- excitation fluorescence detection of the OH (OD) product provide a method of determining the symmetry of the HOCl excited state.

Experimental The apparatus used in these experiments is similar to that described previously for the 248 nm photolysis experi-

ments.'O*" The major changes from the previous configu- ration were that an N d : YAG laser (Spectron SL402) provided ca. 5 mJ per pulse at 266 nm, which was focused to a 1 mm diameter spot at the molecular beam 5 mm from the nozzle tip. A half-wave plate was used to set the 266 nm pho- tolysis radiation to be vertically or horizontally polarized within the vacuum chamber. Owing to a problem with the Nd : YAG laser system the degree of polarisation of the 266 nm radiation was only ca. 0.95. This incomplete polarization was taken into account in the subsequent analysis of the recorded OH LEF spectrum.

The tunable probe laser radiation was provided by a JK2000 Nd : YAG pumped dye laser system doubled by an INRAD Autotracker 11. An EG&G 9650 Digital delay gener- ator provided the necessary trigger pulses to produce a delay of ca. 30 ns between the pump and probe laser pulses. The timing jitter from the delay generator is stated as <75 ps and the jitter associated with the triggering of the two Nd : YAG Q-switches was <1 ns. The probe laser beam was collinear and counter-propagating with the pump beam and had a diameter of ca. 3 mm at the interaction region. The fluores- cence was detected at right angles to the laser and molecular beams. The single collection lens imaged a 12 mm diameter region on to the PMT tube.

The vapour above the HOCl or DOCl aqueous solutions (made as described previously") was expanded through a 500 pm glass nozzle to produce the molecular beam. The nozzle was formed by drawing down a 5 mm diameter glass tube to the required diameter hole. The base pressure in the vacuum chamber was 1 x mbar. When the experiment was running the chamber pressure was 1 x mbar with ca. 10 mbar behind the nozzle. No fluorescence was observed with a pure water sample and the signals recorded from the HOCl samples were linear in the 266 nm laser intensity.

Great care was taken to ensure that the OH fluorescence signals were linear with probe laser power. The probe laser energy was < 1 pJ per pulse at the molecular beam and verti- cally polarized. The fluorescence signals were corrected for variations in the laser powers and the sample vapour pres- sure during the scans. Corrections were also made for the change in detection efficiency across the band due to the filters and the PMT response. A check that all these correc- tions had been performed correctly was made by comparing the populations derived from corresponding transitions in the P and R branches of the OH spectrum, which should be the same once the rotational alignment has been taken into account.

Fig. 1 shows the (0-0) laser excitation fluorescence band

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1624 J. LHLM. 3UL. FAKAUAY IKANS., IYYJ, VWL. 8 Y

J I

Y L L

I I I I I I I I I I I

306.0 307.0 308.0 309.0 31 0.0 31 1 .O wavelength/nm

Fig. 1 HOCl. Lower trace: OD spectrum obtained from the photolysis of DOCl.

Upper trace: (0-0) band of the laser-excitation fluorescence spectrum of OH (Z-II) transition obtained following the dissociation of

for OH (upper trace) and OD (lower trace) of the (Z-II) tran- sition recorded between 306 and 312 nm. The spectra were obtained by recording a series of overlapping sections.

Fragment Rotational Alignment The line strengths of the OH rotational transitions in the laser-excitation fluorescence spectrum are sensitive to the quadrupole moment alignment parameter, Ah2), of the OH photofragment distribution. 1 2 , 1 The Ah2)(J) moment can be estimated from the ratio of the intensities of the main tran- sition and accompanying satellite line originating from the same lower level. However the poor resolution of the dye laser used (0.5 cm-’) and the relatively large Doppler width (0.7 cm-’) arising from the high degree of transitional excita- tion of the OH radical means that it was very difficult to resolve the satellite transitions other than in the R, branch.

The satellite R, l(J) transitions are essentially ‘Q-like’ and any bulk alignment will have a different effect on these tran- sitions than on the main transition. The effect of alignment on the intensity is small and so there will always be a large uncertainty on the value of Ab2) = 4/5fii(02).13 The lowest ground-state rotational level for which the satellites can be resolved in the OH spectrum is J = 3 and by J = 8 the inten- sities of the lines are too small to allow an estimate of 8302) to be made; the values obtained are given in Table 1. There is no obvious J dependence of the alignment and the average value is pg(02) = -0.28 f 0.1 for OH. In the rest of the analysis this value was assumed for all the lower levels, i.e. for all the spin-orbit and lambda doublet stages of OH. The

Table 1 for OH from the photolysis of HOCl at 266 nm

Experimental values of pb’’(O2) for a number of values of J”

3.5 4.5 5.5 6.5 7.5 8.5

- 0.25 - 0.34 - 0.24 - 0.26 - 0.34 - 0.28

measured polarization ratio of the 266 nm radiation was taken into account in the evaluation of 8302).

In principle the spectra recorded with the 266 nm photoly- sis radiation polarized vertically and horizontally can be combined nominally in a 1 : 2 ratio to eliminate alignment effects. However, while this technique was used to minimise the effects of alignment on the actual populations, the deter- mination of Pg(02) from the differences between the two sets of spectra was too susceptible to random and possible system atic errors and the results obtained were not reliable. The overall rotational level populations and the spin-orbit state population ratio are not very sensitive to the assumed value of 8:(02), but the lambda doublet population is sensitive to the exact value chosen.

The O D fluorescence spectrum has many more blended and unresolved lines and only the Q1(9)/Q2’(9) lines were suf- ficiently resolved to calculate a value of pE(O2). This pair of transitions gave &02) = -0.2, similar to the value obtained from the HOCl photolysis.

Fragment Population Distributions The rotational state populations can be derived for both OH spin-orbit states and lambda doublet levels using the weighted sum of the spectra recorded with the 266 nm radi- ation vertically and horizontally polarized. A similar analysis of the individual spectra using Pg(02) = -0.28 for all the energy levels gave results identical within the error bars. Not all the populations can be obtained because of blended lines in the spectra.

The rotational state population distributions obtained are shown in Fig. 2 and 3 for OH (from HOCl) and O D (from DOCl), respectively, as a function of the OH/OD X211 quantum number N . The quantum number N , which is well defined in Hund’s case (b), includes the nuclear rotational angular momentum and the electron orbital angular momen- tum. N does not include the electron spin angular momen- tum and so the use of N in Fig. 2 and 3 facilitates comparison between the population distributions of the two OH spin- orbit states. The OH population distribution was fitted quite well by a Boltzmann distribution, as was the case for the OH

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

N"

1625

2 4 6 8 10

N"

0

N" N"

Fig. 2 OH rotational state population distributions from the photolysis of HOCl at 266 nm. For OH: (a) 2113/2 spin-orbit state and n(A') lambda doublet, (b) 2113/2 spin-orbit state and ll(A") lambda doublet, (c) 2111,2 spin+rbit state and n(A) lambda doublet, (d) 2n1/2 spin-orbit state and n(A) lambda doublet. The solid lines indicate Gaussian fits to the populations, the parameters for which are given in Table 2. The error bars are +_ 3a.

population distribution from the 248 nm photolysis." The Gaussian width parameter, AN, is similar for all the However, the OD product distributions are not at all well spin-orbit and lambda doublet states (ca. 3.3 for OH and 5 represented by a Boltzmann distribution, as was also for OD), but the n(A') lambda doublet states are consistently observed with the 248 nm photolysis." Both sets of results more rotationally excited and more populated than the n(A") can be fitted by Gaussian distributions shown by the solid lambda doublet states. This applies to both OH and OD and lines in Fig. 2 and 3; the parameters of the Gaussians are is seen in both the spin-orbit states. The OD distributions given in Tables 2 and 3. are shifted to larger N and are slightly wider than the corre-

Table 2 Parameters of the Gaussian fits to the OH rotational state population distributions from the photolysis of HOCl at 266 nm

P ( N ) = A exp- -

state A A AN (E,,,)fcm-

2113/2 spin-orbit state, n(A) lambda doublet 55.9 3.9 3.3

'll 112 spin-orbit state, n(A") lambda doublet 9.1 0.5 5.3

2113/2 spin-orbit state, ll(A") lambda doublet, 32.4 0.1 5.8 'll li2 spin-orbit state, ll(A') lambda doublet 24.0 4.0 3.0

' l l 3 / 2 spin-orbit state, sum of lambda doublets 80.5 3.3 3.6

550 420 400 320 500

Table 3 Parameters of the Gaussian fits to the OD rotational state population distributions from the photolysis of DOC1 at 266 nm

P ( N ) = A e x p - (NLiN>' -

2113,2 spin-orbit state, n(A') lambda doublet 49.5 6.5 4.6 2113,2 spin-orbit state, n(A") lambda doublet 27.7 4.2 5.2 2111/2 spin-orbit state, ll(A') lambda doublet 25.4 6.9 4.2 2111/2 spin-orbit state, n(A") lambda aoublet 7.0 5.1 5.1 2113/2 spin-orbit state, sum of lambda doublets 74.4 5.8 5.0

660 440 5 50 420 590

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1626 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

(d ) -

30

t .s 20 (I)

3 a Q Q)

-

.- + m 10 2 -

0

N"

2 4 6 8 10 12 14

N"

30

C

(I)

3 Q

Q 0)

.g 20 -

.- w m 10 !? -

0

' I ' I ' I ' I ' I ' I ' I

P

2 4 6 8 10 12 14

N" Fig. 3 OD rotational state population distributions from the photolysis of DOCl at 266 nm. For OD: (a) 2113j2 spin-orbit state and II(A') lambda doublet, (b) 2113,2 spin-orbit state and n(A") lambda doublet, (c) 21'11/2 spin-orbit state and ll(A') lambda doublet, ( d ) 2111/2 spin-orbit state and l l ( A ) lambda doublet. The solid lines indicate Gaussian fits to the populations, the parameters for which are given in Table 3. The error bars are f 3a.

sponding OH distributions. However, the average rotational energy is similar in both the OH and OD fragments.

If the populations are summed over the lambda doublet states then the distributions for each of the spin-orbit com- ponents given in Fig. 4 are obtained. The fit shown for the OH(211,i2) state is the same as that for the OH(211,12) state simply scaled. This shows that to a good approximation the relative rotational population distributions averaged over the lambda doublet levels for the two spin-orbit states are the

same, with the OH(2113i2) being the most populated. The same is probably true for the OD distribution, but the large number of blended lines means that there are fewer points in this plot.

Spin-Orbit State Populations Both photolysis products can be formed in one of two spin- orbit states, Cl(2P3,2, 2Pli2) and OH(2113i2, 2111,2), separated by 880 and 126 cm-', respectively. In the experiments

90 1

N"

c 60

Q)

2 30

20

10

/ \

N"

Fig. 4 (a) OH rotational states population distributions from the photolysis of HOCl at 266 nm. B, 2113/2 spin-orbit [populations summed over II(A') and II(A") lambda doublet states]. A, 2111,2 spin-orbit state [populations summed over H(A') and n(A") lambda doublet states]. The solid line indicates a Gaussian fit to the 211312 populations. The dashed line is a linear scaling of the Gaussian, by a factor of 0.39. The parameters of the Gaussian fit are given in Table 2. (b) OD rotational state population distributions from the photolysis of DOCl at 266 nm. ., 211,/2 spin-orbit state [summed over II(A') and n(A) lambda doublet states]. A, 2111,2 spin-orbit state [summed over ll(A') and n(A) lambda doublet states]. x , 2111/2 spin-orbit state and II(A') lambda doublet state. The solid line indicates a Gaussian fit to the 2113/2 populations. The dashed line is a linear scaling of the Gaussian, by a factor of 0.40. Insufficient data are available for a comparison to be made between the scaled Gaussian and the populations represented by triangles. The parameters of the Gaussian fit are given in Table 3.

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1627

described in this work the OH states can be observed, but the chlorine spin-orbit state cannot be observed. The relative degeneracy of the rotational levels of the OH spin-orbit states would indicate a 2113/2 : 2171,2 population ratio of ( N + l)/N, which tends to 1 for high N levels. The observed ratio of the spin-orbit state populations summed over the lambda doublet states is approximately 2. If the OH(21131?) and Cl(2P312) spin-orbit states are correlated, then the ratio of degeneracies becomes 2(N + l)/N, which has a high-N limit of 2, which is consistent with the observed spin-orbit ratio. At present the simple Franck-Condon theory used to describe photodissociation of non-linear triatomics has not been extended to cover electronic fine-structure states for OH(217) and Cl('P), unlike for the photodissociation of water. l4

Lambda Doublet State Populations The lambda doublet state populations can be probed even with a low-resolution laser system because the rotational selection rules for a 2C +- 'n transition mean that the Q branches probe only the II(A") lambda doublet component while the P and R branches probe the other n(A') com- ponent. The lambda doublet states are distinguished by their symmetry with respect to reflection in the plane of rotation at high J." Fig. 5 gives the ratio (Q - R)/(Q + R), the degree of electron alignment (where Q and R represent the line inten- sities for the different transitions originating from levels with the same value of N"), as a function of 1" for the spin-orbit states of OH and OD. As can be seen, the l7(A') component is favoured increasingly at higher rotational quantum number.

For the 2111,2 state at low N" the ratio is close to zero, but at higher N" the ratio tends towards the high-1 limit for com- plete alignment of -1. However, for the 2173/2 state the

a -0.5

0.3

0.1

-0.1

4 -0.3 a -0.5

-0.7

-0.9

t I \

1 , , , , I , I , 1 2 3 4 5 6 7 8 9 10

J"

high1 limiting value is only -0.7 for both OH and OD, within experimental error.

Discussion Despite the presence of other thermodynamically favourable dissociation pathways (to form H + OCl or 0 + HCl) the photodissociation of HOCl at 266 nm is expected to produce exclusively C1 + OH. Gas-phase photolysis of HOCl in the wavelength region 290-390 nm shows that channels other than C1 + OH contribute <5% of the photolysis products.16 Experiments that use OH laser-excitation fluorescence to monitor the reaction are of course only sensitive to this path way.

The low pressure in the molecular beam and the short timescale of the OH detection following photolysis means that interference from subsequent bimolecular reactions of the C1 atom, for example the reaction of C1 atoms with HOC1, can be neglected.

C1+ HOCl -, C1, + OH

Similarly the loss of OH via reaction with HOCl can simi- larly be disregarded.

OH + HOCl- H 2 0 + OCl

The local pressure in the molecular beam is sufficiently low that even with the high velocity of the OH fragment rotation- al relaxation of the OH by inelastic collisions with H 2 0 or HOCl, does not occur.

The lack of any observable transitions from OH(u = 1) within our signal-to-noise ratio, together with the relatively cold rotational distribution, indicates that the majority of the energy released in the photolysis appears as relative trans- lational motion, giving a velocity of up to 4OOO m s-'. The equilibrium OH bond length in the parent molecule is 0.975

0'3i,i- 0.1

-0.l j '

4 -0.3 4 -0 -0'51 7

V."

0.1 !i -O, '1 \

\ 2 -0.3 -0.5-

-0.7-

0 2 4 6 8 1 0 1 2 1 4 . -0 9-1

J" 3

Fig. 5 Degree of electron alignment (DEA) for OH and OD from the photolysis of HOCl and DOC1 at 266 nm. (a) OH (2113/2) spin-orbit state, (b) OH ('lll/J spin-orbit state, (c) OD (2r13/?) spin-orbit state, ( d ) OD ('rI1,J spin-orbit state. The solid lines are scaled fits using the expression of Andresen and Rothe,lg with high-1 limiting values of (a) -0.73, (b) - 1.00, (c) -0.69, (d) - 1.00. The experimental error bars indicate 30.

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1628 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

A" and is very similar to the bond length of the ground state of the OH radical (0.97 A); the exclusive production of the ground vibrational state of OH(X211) is therefore not sur- prising. The ab initio calculations on HOCl show that the excited-state surfaces are bound in the OH coordinate with the same OH equilibrium bond length as in the HOCl ground state. These surfaces are, however, strongly repulsive along the OC1 coordinate; such a dissociation path will give OH(X 'll, v = 0) products as observed experimentally. Con- sistent with this model no vibrationally excited products are observed in the photolysis of DOCl at 266 nm and similarly no vibrationally excited OH was observed from the photoly- sis of HOCl at 248 nm.

The dissociation energy of HOCl, for the production of OH(211) and C1(2P), is 21 100 cm- ' (252.7 kJ mol- '). With a photon energy of 37600 cm-', at 266 nm, this implies that the amount of energy available for partitioning amongst the fragments, Eavail, is ca. 16500 cm-l. The proportion of the total available energy deposited into OH fragment rotation is 2.5%. For DOCl the dissociation energy is ca. 20800 cm-' (248.9 kJ mol- '), and this leads to a value of Eavail of 16 800 cm-', with 3.1% of this directed into OD rotation. The modified impulsive model' predicts slightly more rotational excitation of OH (ERoJEavai, = 3.9%) but suggests that the O D should have twice the rotational energy of OH (ERoJEavail = 7.3%) in contrast to the observation of similar average rotational energies in OH and OD. So while the impulsive model gives the correct order of magnitude for the release of energy into fragment rotation, it does not accu- rately reflect the role of the mass change in going from OH to OD.

The most probable origin of the OH rotational energy is transfer of zero-point vibrational energy of HOCl (the HOCl angle bending and Cl-0 stretching vibrations) into OH rotation. These two vibrations will give rise to the rotation of the OH fragment in the plane of the molecule. The calcu- lations of Nambu and I ~ a t a ~ ' ~ show that the angle-bending potential-energy curve for the 2 'A' excited state of HOCl is quite isotropic near the ground-state equilibrium angle with a small preference for the linear configuration. The generalised Franck-Condon model derived by Morse and FreedIg for the photodissociation of a bent triatomic molecule should be applicable for HOCl. In the Franck-Condon limit with no 'exit-channel coupling' the Gaussian distribution of parent bending vibrational momentum is transformed into a Gauss- ian diatomic fragment rotational distribution.

The experimental distributions, Fig. 2 and 3, are qualitat- ively similar to the calculated distributions obtained for a transition moment in the plane (Morse and Freed,Ig Fig. 4). This is consistent with the ab initio calculations which place the transition moment for the 1 'A' + 2 'A' transition in the molecular plane and making an angle of 80" with the C10 bond. The parameters used by Morse and Freed for HOCl give for the dissociation of a non-rotating molecule (the centre of the Gaussian) = 0.5, and AN (the width of the distribution) = 3.12. This ignores the difference between N and J , which seems justified since the population distribu- tions are very similar for the two spin-orbit states. The observed AN is remarkably close to that given by the Franck-Condon model, but the average degree of excitation is slightly greater than predicted.

Assuming that the dissociation takes place only from JHoCl = 0 and ignoring any coupling with the electronic degrees of freedom and any torque in the upper state, the Franck-Condon distribution can be written as,2o

with abend the HOCl (or DOC1) harmonic bending wavenum- ber and 4 the harmonic bending wavefunction. This expres- sion also assumes that the Jacobi coordinate angle is almost the same as the valence bending angle and that the normal mode for the bending motion is largely the angle-bending coordinate. This last assumption is questionable for HOCl and the more so for DOCl as the v2 and vg modes are very close in frequency.

The effective mass, m, is defined by

where re and Re are the equilibrium Jacobi coordinates for HOCl (re is the OH bond length). The full Franck-Condon probability distribution has an extra highly oscillatory factor, however, the inclusion of stretch-bend coupling2' and the averaging over K and J states of the parent HOCl or DOCl molecule tends to smooth over these oscillations.

As the reduced mass of the C1, OH pair is so much larger than the OH reduced mass eqn. (2) is dominated by the first term so that

m POH (3) Comparison with the Gaussian expression given in Table 2 shows that the ratio of the widths of the OH and OD Gauss- ian distributions should depend only on the ratio of the OH and O D reduced masses, i.e.

(4)

The quarter power arises from the fact that both the bending frequency and the effective mass depend on the OH (OD) reduced mass and both terms enter into the bending wave- function expression, eqn. (1) and (2).

Eqn. (4) predicts a ratio of ca. 1.17. Experimentally the observed widths of the OD distributions are larger than the widths of the corresponding OH distributions, but by more than predicted by eqn. (4). In fact it seems that the widths are more nearly proportional to the square root of the reduced mass rather than the fourth root.

There is not expected to be significant rotational cooling of the HOCl in the expansion, so that dissociation will be occurring from rotationally excited HOCl (at room tem- perature the average JHocl will be ca. 18). The simple Franck- Condon theory predicts a shift in the maximum of the Gaussian as a function of .IHoC, but only weakly, N = 0.5 + 0.0273(JHoC1 + 0.5);" this is insufficient to account for the

observed results. The influence of photodissociation from dif- ferent K states of the HOCl is not clear, but since the experi- ments are conducted with a relatively rotationally warm beam a number of K states would be expected to contribute to the photofragment distributions.

The higher degree of rotational excitation in OH and O D is likely to come from the small additional torque provided by the anisotropy of the excited-state potential.22 This addi- tional torque will also produce a broadening of the Gaussian distribution and may thus account for the larger AN0,:ANOH ratio commented upon previously. Classical trajectory studies, however, gave a distribution that is significantly colder than the observed distribution7 and a full quantum- mechanical study including the effects of the electronic angular momentum is needed.

The two lowest excited states of HOCl result from the population of the anti-bonding 1 la' orbital which mainly consists of the in-plane oxygen 2p orbital. In the absence of other interactions this orbital will remain on dissociation where it was in the HOCl molecule, namely in the HOCl

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1629

plane. If the zero-point bending vibration of HOCl is the main source of OH rotational excitation, then the plane of rotation of the OH radicals will also be in the molecular plane. The photodissociation will therefore favour the OH lambda doublet with the singly occupied pn orbital in the same plane as the OH rotational motion, that is there should be a strong preference (at least at high J ) for the ll(A’) state. This is the experimentally observed preferred lambda doublet level.

For large values of the total angular momentum quantum number, J , the lobe of the orbital containing the unpaired pz electron in the OH fragment lies perpendicular to the bond axis and either parallel to J or in the plane of rotation. If the dissociation is planar and highly exothermic, the direction of the lobe is determined by the stereochemistry of the disso- ciation process. Then the significant preference for the n(A) state which has the lobe in the plane of rotation, i.e. perpen- dicular to J , at least for high J , follows from the argument given above. At lower values of J the degree of electron align- ment is expected to be less even for the same dynamics, because of the interactions between the electron spin angular momentum and the rotational angular momentum.

Consideration of the interaction of the electron spin with the molecular rotation shows that the extent to which the p orbital lobes are oriented is a function of i = [ /B , where B is the rotational constant and is the spin-orbit coupling con- stant. For OH and O D in their X211 states, R = -7.5, A = - 15, respectively. It is important to determine whether the observed dependence of the degree of alignment (DEA) with N , seen in Fig. 5 is due to changes in the chemical dynamics or to incomplete orientation for small J . The dependence of the DEA on J has been obtained by Andresen and R ~ t h e ~ ~ and the predictions are shown by the full lines in Fig. 5.

The experimentally observed degree of alignment for the 2113/2 state approximately follows the curve but deviates slightly from the predicted values at low J . If the curve was followed exactly this would indicate that the dynamics were highly selective towards the II(A’) state and the reduction in the degree of alignment for low J was purely an angular momentum effect. The optimum fit to the 2113/2 DEA is obtained if the curve is scaled to give a high-J limiting value of -0.73. This suggests that there may be some decrease in the dynamical orbital alignment for dissociation pathways that give low fragment rotation. Interestingly for the 211 l!2

state the optimum fit has a high-J limiting value of - 1. This effect is also observed in DOC1, where the optimum fits give high-J limiting values of -0.69 (2113/2) and - 1 (2111,2). Some caution must be taken when interpreting the lambda doublet ratio as it is strongly correlated with the bulk alignment

The 2 : 1 spin-orbit population ratio preference in favour of the 2113/2 state suggests that production of the C1 and OH spin-orbit states is correlated. The present investigation can- not probe the C1 fine structure states but future REMPI studies will be made to investigate the C1 atom 2P1/2 : 2P3/2

population ratio.

P m ) .

Excited-state Symmetry The most recent ab initio studies of HOCl by Nambu and Iwata6s7 predict two transitions in the UV at longer wave- lengths than previously suggested. These are excitation of either a 3a” electron (OC1 z antibonding orbital) or a 10a’ electron (in plane non-bonding C1 3p orbital) to the l la ’ orbital (OCI Q antibonding) with vertical excitation energies of 4.02 eV (308 nm) and 5.16 eV (240 nm) respectively, corre- sponding to upper states of 1 ‘A” and 2 ‘A’ symmetry. A

transition is predicted deeper into the UV at 6.77 eV (183 nm) (2 ‘A” t X ‘A) which is expected to be a double excitation of two 3a” electrons to the l la’ orbital. At 266 nm the tran- sitions to the 1 ‘A” and 2’A” states have similar intensity, approximately three orders of magnitude weaker than the transition to the 2 ‘ A state.

The OH fragment alignment provides an experimental indication of the upper-state symmetry involved in the disso- ciation process. In the high4 limit (for the OH and O D frag- ment rotation) the rapid planar dissociation of HOCl from an A or A” state would give a limiting value for flg(02) of -0.5 and + 1.0, respectively. In this limit

where a is the angle between the HOCl transition moment and the .IOH. The transition moment is in the plane for excita- tion to the A‘ state but perpendicular to the plane for excita- tion to the A” state. While fragment alignment is difficult to determine accurately in these experiments, the results from the HOCl photolysis indicate that the dissociation is pri- marily via the A’ state which is consistent with the results of the most recent ab initio calculations.

The observed value of fli(02) does not reach the maximum alignment possible for this situation. The reduction from the maximum value may be due to a dynamical effect leading to JOH not being perpendicular to the original molecular plane. Alternatively the effective value of Pi(02) observed may be a weighted combination from two pathways having the limiting values of -0.5 and + 1.0. Neglecting any interference effects between these channels the value of -0.28 would indicate that 85% of the absorption at 266 nm goes via the A’ state and 15% via the A” state. This is a higher contribution of the A” state than indicated by the ab initio calculation which gives a ratio of absorption coefficients of 1000 : 1 at 266 nm. Similarly the effective value of Pi(O2) for DOCl would imply a combination of 79% A’ and 21% A’’.

Conclusion In this work the photodissociation dynamics of HOCl and DOCl at 266 nm have been found to be similar to those observed at 248 nm. Taken together with the results of ab initio calculations, the results show that the excitation in this region is predominantly to an A’ state which is strongly repulsive along the 0-C1 bond but fairly isotropic in the bending coordinate in the HOCl dissociative excited state. The OH rotational population distributions are given approximately by Franck-Condon distributions but show that some additional torque is applied to the OH (or OD) fragment on dissociation.

The support of the SERC and the CEC is gratefully acknowl- edged.

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Paper 2/06185F; Received 20th November, 1992

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