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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!