High resolution studies of the 3 band of methyl fluoride in solid para-H2 using a quantum cascade laserA.R.W. McKellar*, Asao Mizoguchi, Hideto KanamoriDepartment of Physics, Tokyo Institute of Technology

*Steacie Institute, NRC

• Rapid vapor deposition technique

• 3 band (C-F stretch, 1049 cm-1); also 23 and CD3F

• FTIR (resolution 0.02 cm-1)

• No rotational structure

• Well-resolved lines for n = 0, 1, 2, 3, 4,… 12, where n = the number of ortho-H2 nearest neighbor molecules

• Pattern of lines depends on ortho-H2 concentration, annealing history of crystal, time, etc.

• 1 to 6 bands

• FTIR (resolution 0.05 – 0.20 cm-1)• Rapid vapor deposition technique

• Perpendicular bands (4 to 6) with K = ±1 selection rules show evidence for free K rotation

• CH3F: A = 5.18 cm-1; B = 0.852 cm-1

• CH3F nuclear spin conversion observed: E (K = 1) to A (K = 0)

• Rate of conversion depends on ortho-H2 concentration

Ar

pH2

Lee, Wu, & Hougen (2008)

• FTIR to monitor crystal growth• Rapid vapor deposition technique• Hamamatsu 9.6 μm DFB quantum cascade laser (QCL)

Our work at Tokyo Tech

QCL coverage:~1036 cm-1 at +40 C~1041 cm-1 at 0 C~1044 cm-1 at -30 C (??)

Rapid scan (500 Hz) of laser (no modulation)

Two channels displayed (and averaged & recorded) on digital scope:

• Main sample channel, sometimes with gas cell for absolute wavenumber calibration

• Secondary etalon channel for relative wavenumber calibration (FSR = 0.01 cm-1)

Wavenumber / cm-1

1039.5 1039.6 1039.7 1039.8 1039.9 1040.0 1040.1 1040.2 1040.3

n = 1 and 0 at ~2 K after annealing:

#091028022

#091028068

CH3F P(5)

Wavenumber calibration using CH3F gas-phase lines

n = 0n = 1

Data point number

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Det

ecto

r si

gnal

(a

c co

uple

d)

-40000

-20000

0

20000

40000

60000

spcc042_raw

Data point number

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

De

tect

or

sign

al (

dc c

oupl

ed)

0

2000

4000

6000

8000

10000

12000

14000

confocal etalon

fsr = 0.0097 cm-1

Waavenumber / cm-1

1037.6 1037.8 1038.0 1038.2 1038.4

De

tect

or

sign

al (

tru

e d

c)

0

10000

20000

30000

40000

50000

60000

70000

80000

linear background slope

Data processing

fitted polynomial background

confocal etalon (fsr = 0.0097 cm-1)

detector zero level

Col 1 vs Col 3

Wavenumber / cm-1

1037.6 1037.8 1038.0 1038.2 1038.4

Abs

orba

nce

0.0

0.2

0.4

0.6

0.8

1.0

Absorbance = ln[I0()/I()]I0() = background spectrumI() = sample spectrum

Resulting calibrated spectrum

Absolute wavenumber scale comes from CH3F gas cell (not present for all spectra)

Motivation• Test QCL in a new spectroscopic application

• Study a quantum crystal (CH3F in para-H2) with high spectral resolution and high time resolution (real-time monitoring)

• Test improvements to the para-H2 conversion apparatus, since CH3F spectrum provides a good measure of residual ortho-H2 concentration (?)

Results• We quickly discover that the laser power is high enough to affect the crystal (P ~ 10 – 30 mW)

• For example, n = 1 or 2 lines disappear after a few seconds of observation (“bleaching” effect)

• So it is necessary to strongly attenuate the laser beam before it enters the cryostat

Results: polarization

• The para-H2 crystal axis is known to be oriented with respect to the substrate• The substrate is oriented at 45° to the incident laser beam• Changing from horizontal to vertical laser polarization therefore probes the orientation of the molecular dipole moment (i.e. the CH3F symmetry axis) with respect to the crystal axis

• We observed NO polarization dependence

Wavenumber / cm-1

1039.4 1039.5 1039.6

0.0

0.1

0.2

0.0

0.2

0.4

0.0

0.5

1.0

1.5

2.0

1040.1 1040.2 1040.3

0.0

0.1

0.2

0.3

Abs

orba

nce

0.0

0.1

0.2

0.0

0.5

1.0

1.5 after 7 K "super"

annealing

after 4.6 K annealing

before annealing

0.0

0.2

0.4

0.6

0.8

0.0

0.2

0.4

1038.7 1038.8 1038.9

0.0

0.1

N = 0 N = 1 N = 2

* * *

Annealing• After deposition at ~2 K, the sample is a polycrystalline mixture of hcp and fcc structures• Annealing at ~4.9 K for 10 minutes converts it to a more nearly ideal hcp single crystal • Super-annealing at ~7 K for 10 seconds is even better(!?)

Wavenumber / cm-1

1040.15 1040.20

5.0 K

4.5 K

4.0 K

3.5 K3.0 K

2.5 K2.0 K

n = 0 temperature dependence

after annealing

Temperature shift coefficient is about -0.0011 cm-1/K for the N = 0 line

N = 0 temperature dependence

after ‘normal’ annealing at 5 K

Wavenumber / cm-1

1040.16 1040.17 1040.18 1040.19 1040.20 1040.21 1040.22

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

0.0

0.5

1.0

1.5

2.0

N = 0N = 0 at 1.8 K after

super-annealing

Profile analysis gives two Lorentzian components:

1040.189 cm-1; width = 0.0065 cm-1 1040.182 cm-1; width = 0.0076 cm-1

which we assign as: K = 0 (ortho-CH3F)

K = 1 (para-CH3F)

Note the slow decay of K = 1 over a period of 6 hours, similar to that observed by Lee, Wu, and Hougen.

Hours after annealing

00.40.736

gas-phase methyl fluoride

ortho-CH3FI = 3/2K = 0, 3, 6, … A symmetry

para-CH3FI = 1/2K = 1, 2, 4, 5, 7, … E symmetry

methyl fluoride in para-H2 crystal

ortho-CH3FI = 3/2K = 0 Energy = 0 cm-1

para-CH3FI = 1/2K = 1 Energy ~ 4.6 cm-1

(Lee, Wu & Hougen)

N = 1 at 1.8 K after

super-annealing

Profile analysis gives three Lorentzian components:1039.483 cm-1; width = 0.0081 cm-1

1039.475 cm-1; width = 0.0089 cm-1

1039.492 cm-1; width = 0.0070 cm-1

which we assign as: K = 0 (ortho-CH3F)

K = 1 (para-CH3F)

fast-decaying metastable component

0 hours after annealing 0.40.736

Wavenumber / cm-1

1039.45 1039.46 1039.47 1039.48 1039.49 1039.50 1039.51

Abs

orba

nce

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

N = 1

N = 2 at 1.8 K after

super-annealing

There is a main line plus a fast-decaying component. Each of these lines seems to have a low frequency (K = 1?) shoulder which decays at a slower rate.Line widths are ~0.0095 cm-1, which is a bit broader than for the N = 0 and 1 lines.

0 hours after annealing 0.40.736

Wavenumber / cm-1

1038.74 1038.76 1038.78 1038.80 1038.82

Ab

sorb

anc

e

0.0

0.1

0.2

0.3

0.4N = 2

N = 3 at 1.8 K after

super-annealing

Super annealing causes a completely new feature to appear in the N = 3 region at 1038.318 cm-1. This new feature is persistent (does not decay), and sharp (~0.0060 cm-1).Both the original and new lines have low frequency shoulders.

Interestingly, N =3 appears to be more persistent than N = 2 over a long period (1 to 3 days).

1038.06 1038.08 1038.10

Abs

orba

nce

0.00

0.05

0.10

0.15

1038.30 1038.32 1038.34

Wavenumber / cm-1

1038.0 1038.1 1038.2 1038.3

Abs

orba

nce

0.0

0.2

0.4

0.6 N = 3

normal annealing

super annealing

Abs

orba

nce

0.00

0.10

0.20

Wavenumber / cm-1

1037.3 1037.4 1037.5 1037.6 1037.7

0.00

0.02

0.04

N = 4

N = 4 at 1.8 K after super-annealingSuper annealing also causes sharp new features to appear in the N = 4 region!

normal annealing

super annealing

N = 5 at 1.8 K

After normal annealing, N = 5 has two components, as observed by Yoshioka & Anderson. But after super annealing N = 5 almost disappears

Wavenumber / cm-1

1036.65 1036.70 1036.75 1036.80 1036.85 1036.90

Abs

orba

nce

0.0

0.1

0.2

0.3

0.4

0.5

0.6

N = 5

normal annealing

Mystery lines:with “N = ½”

These are very sensitive to temperature – they disappear if T is raised from 1.7 K to 2.0 K

Wavenumber / cm-1

1039.75 1039.80 1039.85 1039.90 1039.95

Abs

orba

nce

0.00

0.02

0.04

0.06

0.08

0.10

1039.802

1039.854

1039.897

1039.923

1039.8538 cm-1

0.0065 cm-1

0.0805-0.0085

Wavenumber / cm-1

1039.4 1039.6 1039.8 1040.0 1040.2

Abs

orba

nce

0.00

0.05

0.10

0.15

0.20

0.25

0.30

1039.802

1039.854

1039.897

1039.923

N = 0N = 11039.484

1040.189

1040.1151039.545

Wavenumber / cm-1

1039.4 1039.6 1039.8 1040.0 1040.2

Abs

orba

nce

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8 N = 1N = 0

(1) before bleaching

(2) after bleaching N = 1

(3) 25 minutes later

(4) after re-bleaching N = 1

(5) after bleaching here

“Bleaching” or “Hole-Burning”

Wavenumber / cm-1

1038.8 1039.0 1039.2 1039.4 1039.6

Abs

orba

nce

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3 N = 1

N = 2

(6) before bleaching N = 2

(7) after bleaching N = 2

(8) after bleaching here

“Bleaching” or “Hole-Burning”

Wavenumber / cm-1

1039.4 1039.6 1039.8 1040.0 1040.2

Abs

orba

nce

0.0

0.2

0.4

0.6

0.8

1.0

1.2

N = 1

N = 0

(9) before bleaching

(10) after bleaching N = 0 for 110 s

(11) after bleaching N = 1 for 40 s

“Bleaching” or “Hole-Burning”

Conclusions• No polarization dependence detected• Lines are as sharp as 0.006 cm-1, with Lorentzian shapes• Super annealing!?• K = 1 shoulders are resolved, about 0.008 cm-1 to the red of K = 0• Weak “N = ½” mystery lines

• First(?) hole-burning experiments on a para-H2 matrix isolated sample -- population can be reversibly transferred

• Theoretical calculation of H2-induced vibrational shifts for 3 of CH3F would be very useful!