High resolution studies of the 3 band of methyl fluoride in solid para-H 2 using a quantum cascade laser A.R.W. McKellar *, Asao Mizoguchi, Hideto Kanamori Department of Physics, Tokyo Institute of Technology * Steacie Institute, NRC

Rapid vapor deposition technique 3 band (C-F stretch, 1049 cm -1 ); also 2 3 and CD 3 F 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-H 2 nearest neighbor molecules Pattern of lines depends on ortho-H 2 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 CH 3 F: A = 5.18 cm -1 ; B = cm -1 CH 3 F nuclear spin conversion observed: E (K = 1) to A (K = 0) Rate of conversion depends on ortho-H 2 concentration Ar pH2pH2 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 calibration using CH 3 F gas-phase lines n = 0 n = 1

Data processing fitted polynomial background confocal etalon (fsr = cm -1 ) detector zero level

Absorbance = ln[I 0 ( )/I( )] I 0 ( ) = background spectrum I( ) = sample spectrum Resulting calibrated spectrum Absolute wavenumber scale comes from CH 3 F gas cell (not present for all spectra)

Motivation Test QCL in a new spectroscopic application Study a quantum crystal (CH 3 F in para-H 2 ) with high spectral resolution and high time resolution (real-time monitoring) Test improvements to the para-H 2 conversion apparatus, since CH 3 F spectrum provides a good measure of residual ortho-H 2 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-H 2 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 CH 3 F symmetry axis) with respect to the crystal axis We observed NO polarization dependence

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(!?)

Temperature shift coefficient is about cm -1 /K for the N = 0 line N = 0 temperature dependence after ‘normal’ annealing at 5 K

N = 0 at 1.8 K after super-annealing Profile analysis gives two Lorentzian components: cm -1 ; width = cm cm -1 ; width = cm -1 which we assign as: K = 0 (ortho-CH 3 F) K = 1 (para-CH 3 F) 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

gas-phase methyl fluoride ortho-CH 3 F I = 3/2 K = 0, 3, 6, … A symmetry para-CH 3 F I = 1/2 K = 1, 2, 4, 5, 7, … E symmetry methyl fluoride in para-H 2 crystal ortho-CH 3 F I = 3/2 K = 0 Energy = 0 cm -1 para-CH 3 F I = 1/2 K = 1 Energy ~ 4.6 cm -1 (Lee, Wu & Hougen)

N = 1 at 1.8 K after super-annealing Profile analysis gives three Lorentzian components: cm -1 ; width = cm cm -1 ; width = cm cm -1 ; width = cm -1 which we assign as: K = 0 (ortho-CH 3 F) K = 1 (para-CH 3 F) fast-decaying metastable component 0 hours after annealing

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 ~ cm -1, which is a bit broader than for the N = 0 and 1 lines. 0 hours after annealing

N = 3 at 1.8 K after super-annealing Super annealing causes a completely new feature to appear in the N = 3 region at cm -1. This new feature is persistent (does not decay), and sharp (~ 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). normal annealing super annealing

N = 4 at 1.8 K after super-annealing Super 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 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

“Bleaching” or “Hole-Burning”

Conclusions No polarization dependence detected Lines are as sharp as cm -1, with Lorentzian shapes Super annealing!? K = 1 shoulders are resolved, about cm -1 to the red of K = 0 Weak “N = ½” mystery lines First(?) hole-burning experiments on a para-H 2 matrix isolated sample -- population can be reversibly transferred Theoretical calculation of H 2 -induced vibrational shifts for 3 of CH 3 F would be very useful!