Laser spectroscopy and lifetime measurements of Dy I states
3
1514 OPTICS LETTERS / Vol. 16, No. 19 / October 1, 1991 Laser spectroscopy and lifetime measurements of Dy I states Dmitry Budker, Eugene D. Commins, and David DeMille Department of Physics,University of California,Berkeley,Berkeley,California 94720 Max S. Zolotorev Stanford Linear Accelerator Center,Stanford, California 94309 Received June 10, 1991 The dysprosium spectrum is studied in the range of 273-284 nm by using pulsed-laser excitation in an atomic beam and fluorescence detection. Lifetimes and branching ratios for the decay to the ground term are deter- mined from the analysis of fluorescence signals for 27 high-lying odd-parity levels. In the research reported here, which is part of a pro- ject developed at Berkeley to evaluate feasibility of parity and time-reversal violation experiments with atomic dysprosium,' we study the dysprosium spec- trum in the range of 273-284 nm using pulsed-laser excitation from the ground state in an atomic beam and fluorescence detection (Fig. 1). Of the 27 observed transitions, approximately half were identified as transitions between the ground state and the odd-parity excited states listed in tables of dysprosium levels. 2 All observed transi- tions have been observed before in the emission spectrum of an electrodeless lamp with high- frequency excitation 3 and (with high intensity) in the absorption spectrum in a high-temperature King furnace. 4 ' However, their observation in many cases did not lead to identification of the cor- responding energy levels, apparently because under the conditions of Refs. 3-5 neither the upper nor the lower level of the transition was known a priori. The conditions of the present research ensure that the observed transitions originate from the ground state (4f1' 0 6s 2 5I8). This, along with the known tran- sition energies, allows one to extend the level scheme of dysprosium with a few new states. The observed temporal evolution of fluorescence signals corresponding to decay of the excited states after pulsed excitation was used to determine the natural lifetimes of these states. To our knowledge, the lifetimes of these levels have not been measured before, although there exist lifetime data on a num- ber of other dysprosium states. 69 The experiment employed orthogonal atomic and laser beams and fluorescence detection in the third orthogonal direction. A dysprosium beam was pro- duced in an effusive source that incorporated a tan- talum oven resistively heated with tantalum wire heaters to -1550 K (corresponding a vapor pressure of -0.1 Torr). The atomic beam density in the in- teraction region -10 cm downstream from the oven was -5 x 109 cm- 3 . UV light pulses were produced by doubling the output of a Quanta Ray PDL-2 dye laser in an angle-tuned KD*P crystal. The dye laser was operated with Fluorescein 548 and pumped at a 10-Hz rate by the second harmonic of a DCR-2 Nd:YAG laser (pulse duration 6-7 ns). The dye laser was operated without an intracavity 6talon, and the radiation linewidth before the dou- bling crystal was -0.5 cm-'. After the doubling crystal, the UV radiation was isolated with a har- monic separator prism. Typical UV power was -1 mJ/pulse, and the laser beam diameter in the in- teraction region was -2-3 mm. Atomic fluores- cence originating from the excited-state decay to the ground-term levels was detected by an EMI 9750 QA photomultiplier with a quantum efficiency of -20% in the relevant spectral interval. The photo- multiplier was situated -20 cm from the interaction region. The light-collection solid angle was limited by a diaphragm with an area of 0.3 cm 2 in front of the photomultiplier input window, which was used to keep the signal at the level that ensured the lin- earity of the photomultiplier response. In order to suppress background from cascade fluor- escence, a UV-transmitting, visible absorbing filter (Corning 7-54) was installed in front of the photo- multiplier. The photomultiplier output was directly connected to a 50-fQ input of a Tektronix 2430A digital oscilloscope that was used for digitizing, averaging (typically for a few seconds), and record- ing of the fluorescence decay profiles. Since the experiment is sensitive to fluorescence at the wave- length of excitation, precautions were taken to mini- mize scattering of the laser light into the photomultiplier. The nonresonant signal from the residual scattered light was faster and typically smaller than the fluorescence signal (see Fig. 2) and did not depend on whether the atomic beam was on. Results of the spectrum scan are summarized in Table 1. Transition energies were first estimated with an uncertainty of 0.8 cm-' from the setting of the dye-laser diffraction grating, which had been calibrated with a monochromator. The list of ob- served transition energies was compared with lists of transition energies measured in Refs. 3 and 5 with typical uncertainties of a few hundredths of an inverse centimeter. This comparison led (with a single exception denoted in the table) to an unam- 0146-9592/91/191514-03$5.00/0 ( 1991 Optical Society of America
Transcript of Laser spectroscopy and lifetime measurements of Dy I states
1514 OPTICS LETTERS / Vol. 16, No. 19 / October 1, 1991
Laser spectroscopy and lifetime measurements of Dy I states
Dmitry Budker, Eugene D. Commins, and David DeMille
Department of Physics, University of California, Berkeley, Berkeley, California 94720
Max S. Zolotorev
Stanford Linear Accelerator Center, Stanford, California 94309
Received June 10, 1991
The dysprosium spectrum is studied in the range of 273-284 nm by using pulsed-laser excitation in an atomicbeam and fluorescence detection. Lifetimes and branching ratios for the decay to the ground term are deter-mined from the analysis of fluorescence signals for 27 high-lying odd-parity levels.
In the research reported here, which is part of a pro-ject developed at Berkeley to evaluate feasibility ofparity and time-reversal violation experiments withatomic dysprosium,' we study the dysprosium spec-trum in the range of 273-284 nm using pulsed-laserexcitation from the ground state in an atomic beamand fluorescence detection (Fig. 1).
Of the 27 observed transitions, approximatelyhalf were identified as transitions between theground state and the odd-parity excited states listedin tables of dysprosium levels.2 All observed transi-tions have been observed before in the emissionspectrum of an electrodeless lamp with high-frequency excitation3 and (with high intensity) inthe absorption spectrum in a high-temperatureKing furnace.4 ' However, their observation inmany cases did not lead to identification of the cor-responding energy levels, apparently because underthe conditions of Refs. 3-5 neither the upper nor thelower level of the transition was known a priori.The conditions of the present research ensure thatthe observed transitions originate from the groundstate (4f1'0 6s2 5I8). This, along with the known tran-sition energies, allows one to extend the levelscheme of dysprosium with a few new states.
The observed temporal evolution of fluorescencesignals corresponding to decay of the excited statesafter pulsed excitation was used to determine thenatural lifetimes of these states. To our knowledge,the lifetimes of these levels have not been measuredbefore, although there exist lifetime data on a num-ber of other dysprosium states.6 9
The experiment employed orthogonal atomic andlaser beams and fluorescence detection in the thirdorthogonal direction. A dysprosium beam was pro-duced in an effusive source that incorporated a tan-talum oven resistively heated with tantalum wireheaters to -1550 K (corresponding a vapor pressureof -0.1 Torr). The atomic beam density in the in-teraction region -10 cm downstream from the ovenwas -5 x 109 cm-3 . UV light pulses were producedby doubling the output of a Quanta Ray PDL-2 dyelaser in an angle-tuned KD*P crystal. The dyelaser was operated with Fluorescein 548 and
pumped at a 10-Hz rate by the second harmonic of aDCR-2 Nd:YAG laser (pulse duration 6-7 ns). Thedye laser was operated without an intracavity6talon, and the radiation linewidth before the dou-bling crystal was -0.5 cm-'. After the doublingcrystal, the UV radiation was isolated with a har-monic separator prism. Typical UV power was-1 mJ/pulse, and the laser beam diameter in the in-teraction region was -2-3 mm. Atomic fluores-cence originating from the excited-state decay to theground-term levels was detected by an EMI 9750QA photomultiplier with a quantum efficiency of-20% in the relevant spectral interval. The photo-multiplier was situated -20 cm from the interactionregion. The light-collection solid angle was limitedby a diaphragm with an area of 0.3 cm2 in front ofthe photomultiplier input window, which was usedto keep the signal at the level that ensured the lin-earity of the photomultiplier response. In orderto suppress background from cascade fluor-escence, a UV-transmitting, visible absorbing filter(Corning 7-54) was installed in front of the photo-multiplier. The photomultiplier output was directlyconnected to a 50-fQ input of a Tektronix 2430Adigital oscilloscope that was used for digitizing,averaging (typically for a few seconds), and record-ing of the fluorescence decay profiles. Since theexperiment is sensitive to fluorescence at the wave-length of excitation, precautions were taken to mini-mize scattering of the laser light into thephotomultiplier. The nonresonant signal from theresidual scattered light was faster and typicallysmaller than the fluorescence signal (see Fig. 2) anddid not depend on whether the atomic beam was on.
Results of the spectrum scan are summarized inTable 1. Transition energies were first estimatedwith an uncertainty of 0.8 cm-' from the setting ofthe dye-laser diffraction grating, which had beencalibrated with a monochromator. The list of ob-served transition energies was compared with listsof transition energies measured in Refs. 3 and 5with typical uncertainties of a few hundredths of aninverse centimeter. This comparison led (with asingle exception denoted in the table) to an unam-
0146-9592/91/191514-03$5.00/0 ( 1991 Optical Society of America
October 1, 1991 / Vol. 16, No. 19 / OPTICS LETTERS 1515
* I,~~~~~~~~~~,,
.,, /,J6
' J-7: /
J=6J=7
(6
H)5d6s2 7
H H
Odd
Fig. 1. Partial level scheme of Dy i. Only levels withJ = 6-10 are shown. Solid line, laser excitation; dottedlines, fluorescence channels to which the detection systemwas sensitive.
biguous correspondence between each of the transi-tions observed here and one of the lines previouslyobserved in emission and absorption. Thus the ob-served transitions are labeled in the first column ofTable 1 with the more precise transition energiesfrom Refs. 3 and 5. Some of the listed transitionshave been identified in Refs. 3 and 5 as transitionsfrom the ground 4f106s2
'8 state. The upper-stateclassifications for these transitions, as determinedin Refs. 3, 4, and 10 and compiled in Ref. 2, aregiven in the second column of the table. Questionmarks denote tentative configurations. Becausesignals resulting from absorption from low-lying ex-cited states are suppressed by Boltzmann factors(-2 x 10-2 for the lowest level 4f 106s2 5I7) and noneof the transitions from excited states of either par-ity predicted from the level scheme2 has been ob-served in the present research, it is most likely thatall transitions observed here correspond to transi-tions from the ground state. Multipole transitions,which in principle could be excited with a high-power laser pulse, are excluded given the identifica-tion discussed above of transitions observed by uswith those observed in nonlaser experiments.3 5
Therefore the transitions observed here are electricdipole transitions, whose selection rules require thatthe upper states be of odd parity and have total an-gular momentum J = 7, 8, or 9.
Results of the lifetime measurements are given inthe third column of Table 1. To obtain the lifetimefrom a fluorescence decay curve, the following pro-
Table 1. Observed Transitions, Upper-Level Lifetimes, and Estimated Relative Integral Fluorescences
Transition Energy (cm'1) Upper Level Lifetime (ns) Integral Fluorescence (Relative Units)
35,237.79 880(80) 835,354.89 710(50) 1035,370.43 545(40) 135,431.65 530(20) 135,450.10 4f'0 (5I)5d6p? J = 8 207(10) 435,518.20 4f' 0(5I)6s7p J= 8 101(10) 135,580.70 4f10(5I)6s7p J = 9 98(10) 535,777.78 J = 8 221(20) 135,894.35 J = 8 188(10) 435,899.88 J = 8 145(10) 235,955.77 86(10) 136,197.98 290(20) 136,248.00 J = 7 151(10) 136,260.16 4f' 0 (5I)5d6p? J = 9 79(10) 136,288.46a] 4f'0 (5I)5d6p? J = 8
103(10) 336,288.63aJ36,316.36 4f'0(5I)5d6p? J = 7 182(20) 136,332.67 225(20) 136,342.79 230(10) 436,384.76 240(10) 336,436.77 119(10) 136,440.19 4f'0(5I)6s7p J = 9 156(10) 236,441.99 J = 7 219(40) 136,534.01 J = 8 121(10) 236,541.74 128(10) 836,546.71 4f'0(5I)6s7p J = 8 310(30) 136,589.96 535(30) 436,618.30 J = 8 302(30) 1
aThese lines were not resolved in the present experiment. The fluorescence decay was well described with a single exponential.
E, 104 Cm- 141-
3
2
0
Even
1516 OPTICS LETTERS / Vol. 16, No. 19 / October 1, 1991
ground-term levels. Small branching ratios for de-cay into the ground term for a number of excitedstates implies that these states decay preferentiallyinto excited even-parity terms with the lowest levelat 17,514 cm-'.
In summary, we have measured the lifetimes of anumber of odd-parity states of atomic dysprosium.These states include both classified ones and the
B levels that extend the present dysprosium levelscheme. Information was also obtained on thebranching ratio of the decay to the ground term.
0 ) 100 200 300 400 500 600 700 800 The authors acknowledge useful communicationswith J.-F Wyart. This research has been sup-
ns ported by National Science Foundation grant PHY-Typical fluorescence decay curves. Curve A, 8721947.
nonresonant signal; curve B, signal on resonance(35,894.35 cm-'); curve C, difference between curves Band A.
cedure was used. First, the nonresonant part of thesignal was subtracted from the signal on resonance(see Fig. 2). An exponential was then fitted to thetail of the resulting curve. A correction to the life-time obtained from the fit owing to the finite re-sponse time of the detection system was estimatedby modeling the signal as the convolution of a purelyexponential decay with the detection system re-sponse to a short pulse. The response function wastaken to be the nonresonant (scattered-light) signal.The largest corrections had to be made for the short-est lifetimes, for which they did not exceed 10%.The quoted errors are mainly due to uncertainties ofthe fitting procedure. The last column of Table 1shows the integral under the fluorescence curve.For data taken on the same resonance at differenttimes during the experiment, the measured life-times were the same within experimental uncer-tainties, while amplitudes varied within a factor of 2(presumably owing to variations in dysprosium den-sity). Since with the laser power densities utilizedin this experiment the excitation transitions shouldhave been well saturated and only UV light was de-tected, the integral fluorescence characterizes thebranching ratio of the excited-state decay to the
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40
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E -60-80
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Fig. 2.
