Nuclear Spin Symmetry Conservation Studied for Symmetric Top Molecules (CH₃D, CHD₃, CH₃F, and CH₃Cl) in Supersonic Jet Expansions

Abstract

We report results on nuclear spin symmetry conservation studied by high resolution spectroscopy of relative line intensities for the A and E nuclear spin isomers of symmetric top molecules CHD 3 , CH 3 D, CH 3 F, and CH 3 35 Cl in supersonic jet expansions with He and Ar as carrier gases. Infrared absorption spectra were measured around 3000 cm −1 by an infrared (lead salt) diode laser and a continuous wave IR-OPO (infrared optical parametric oscillator) locked to a frequency comb. A detailed analysis of the R(2)-line intensities of the CH-stretching fundamental shows that nuclear spin symmetry is conserved for CHD 3 , CH 3 F, and CH 3 35 Cl during the expansion. For CH 3 D, a small contribution from nuclear spin symmetry relaxation cannot be excluded completely under our experimental conditions.

High resolution FTIR and diode laser spectroscopy of trifluoromethylacetylene and tetrafluoromethane in a supersonic jet expansion

“…The meta (A) isomer relaxes towards J = 0, the ortho (F) isomer to J = 1, and the para (E) isomer to J = 2 without interconversion [24,27,[75][76][77]. Similar results are obtained for the methane isotopomers CH 3 D and CHD 3 , and for CH 3 F, CH 3 Cl [20], and NH 3 [45]. For H 2 O it is found that at high dilution in the expansion (with Argon and with paramagnetic Oxygen) nuclear spin symmetry is conserved, whereas at high concentrations with possibly substantial cluster formation in the jet some interconversion between nuclear spin isomers occurs [139,140] (see also the recent review [141]).…”

“…The resolution of the monochromator around 1000 cm −1 is 1.5 cm −1 , when the widths of the entrance and exit slits are both 1 mm and at least one half of the grating is illuminated by the laser beam. This resolution is good enough to separate [20,21,82]).…”

“…Due to its similarity to methane and the large transition moment for the ν 3 fundamental, CF 4 is a good candidate to investigate nuclear spin symmetry conservation in a molecular beam expansion, similar to CH 4 [24,27,42,43,[75][76][77][78][79], but, as the rotational constant (B = 0.1916 cm −1 ) is more than the 20 times smaller than for CH 4 , much lower rotational temperatures in the molecular beam have to be obtained. Some preliminary results of the present work have been reported in [80][81][82] (see also [20,21]). We also refer to several recent studies of C-C ≡ C bend E 169.2 c.) [50][51][52] Notes: (a) Ref.…”

“…The complete rotational vibrational line analysis of high resolution infrared spectra of polyatomic molecules is of of gas sensing [8], as well as a benchmark for ab initio theory [9][10][11][12] or fundamental investigations of the primary processes of intramolecular quantum dynamics and kinetics [13][14][15][16][17][18][19][20][21]. In this context the combination of high resolution spectroscopic techniques with molecular beams from supersonic jet expansions has played an important role, as the cooling to low effective rotational and vibrational temperatures as well as the reduction of the Doppler width greatly simplifies the analysis of the complex spectra [13,.…”

“…Figure 2. Block diagram of the diode laser spectrometer: A: aperture, AL: alignment laser, BS: beam splitter, Cell CG: cell with calibration gas, Cell Pr: cell with sample gas, D detector, ET etalon, Jet: molecular beam with slit nozzle, MC: monochromator, M: mirror (flipping mirrors are indicated with a direction indicator), OAP: off-axis parabolic mirror, SM: spherical mirror, TDL: diode laser (see also[20,21,82]). …”