Infrared photochromism in a quantum crystal: laser pumping and probing in solid para-H2 doped with CH3F

Solid para-H(2) doped with CH(3)F at 1.8 K is studied in the ν(3) region (～1040 cm(-1)) using a quantum cascade laser source. Residual ortho-H(2) gives rise to distinct spectral features due to CH(3)F-(ortho-H(2))(N) clusters with N = 0, 1, 2, etc. The laser power (～30 mW) is sufficient to significantly affect the sample, enabling novel photochromism experiments to be performed on a solid para-H(2) matrix-isolated sample. It is found that population can be reversibly transferred between the N = 1 line and satellite features close to the N = 0 line.     

Infrared spectra of acetylene dimers and acetylene–nitrogen: (DCCD)2, H-bonded DCCD–HCCH, and DCCD–NN in the 4.1 μm region

Infrared spectra of the weakly-bound T-shaped acetylene dimers DCCD–DCCD and DCCD–HCCH are studied in the region of the DCCD ν₃ fundamental (∼2440 cm⁻¹) using a pulsed supersonic slit-jet expansion and a tunable diode laser probe. The K_a = 0 ← 1 and 1 ← 0 subbands, corresponding to the vibration of the DCCD monomer at the “top” of the T, are analyzed. Compared to the analogous spectrum of HCCH–HCCH, the present results are much less perturbed. The tunneling splitting for (DCCD)₂ in the excited state is determined to be 141 MHz, a big reduction from the previously determined ground state value of 424 MHz. The dimer A rotational constants show a large apparent increase upon vibrational excitation, and we discuss whether this increase is real. The linear DCCD–NN complex is also observed as an impurity in the spectrum, and it too is found to be unperturbed, in contrast with HCCH–NN.     

Spectroscopic observation of nitrous oxide pentamers

Two new infrared bands in the ν(1) fundamental region of N(2)O are observed in a supersonic jet expansion and assigned to nitrous oxide pentamers. Each band is measured using both (14)N(2)(16)O and (15)N(2)(16)O. Although they are similar in appearance, the bands have slightly different lower state rotational parameters, and are thus assigned to distinct structural isomers of the pentamer. Cluster calculations using two N(2)O intermolecular potentials give results in good agreement with the observed spectra, and indicate that the two isomers probably have the same basic structure (which is unsymmetrical), but differ in the alignment (N-N-O or O-N-N) of one or two of the constituent monomers. Calculations using a resonant dipole interaction model also support the proposed assignment and structure. These are the first reported high-resolution spectra for N(2)O pentamers.     

Spectroscopic studies of quantum solvation in 4HeN-N2O clusters

High resolution microwave and infrared spectra of He(N)-N2O clusters were studied in the range N=3 to 12. The apparent cluster moments of inertia increase from N=3 to 6, but then decrease, showing oscillatory behavior for N=7 to 12. This provides direct experimental evidence for the decoupling of helium atoms from the rotation of the dopant molecule in this size regime, signaling the transition from a molecular complex to a quantum solvated system and directly exploring the microscopic evolution of "molecular superfluidity."     

Infrared and Millimeter-Wave Study of the Four Lowest Torsional States of CH(3)CF(3)

An investigation of the torsion-rotation Hamiltonian of CH(3)CF(3) in the ground vibrational state has been carried out using infrared and mm-wave spectroscopy. With infrared Fourier transform spectroscopy, the weak, torsional overtone (v(6) = 2 <-- 0) has been studied leading to the measurement of 382 frequencies between 405 and 440 cm(-1) at a resolution of 0.005 cm(-1). Torsional splittings on the order of 0.03 cm(-1) were observed. With mm-wave methods, a total of 669 rotational transitions between 50 and 360 GHz have been measured at Doppler-limited resolution in the four lowest torsional states v(6) = 0, 1, 2, 3. The experimental uncertainty attained for an isolated line was better than 10 kHz below 150 GHz, and somewhat larger at higher frequencies. For v(6) = 3, torsional splittings as large as 8.7 MHz were observed. The global data set consisted of the current frequency determinations and the 443 measurements with molecular beam, microwave, and mm-wave methods analyzed by I. Ozier, J. Schroderus, S.-X. Wang, G. A. McRae, M. C. L. Gerry, B. Vogelsanger, and A. Bauder [J. Mol. Spectrosc. 190, 324-340 (1998)]. The observation of mm-wave R-branch transitions for v(6) = 1 led to a change in the J-assignment of the forbidden (Deltak = +/-3) transitions reported earlier for this torsional state. A good fit was obtained by varying 24 parameters in a Hamiltonian that represented both the torsional effects and the sextic splittings. In the earlier work, the large reduced barrier height led to high correlations among several of the torsional distortion constants. With the current measurements, many of these correlations are substantially reduced. Improved effective values were determined for the height V(3) of the hindering barrier and the first-order correction V(6) in the Fourier expansion of the potential function. The dipole function which characterizes the transition moment of the torsional overtone (v(6) = 2 <-- 0) can be written as the product of a single effective dipole constant μ(T)(0,eff) and the appropriate off-diagonal matrix element of (1 - cos 3alpha)/2, where alpha is the torsional angle. From an intensity analysis of the infrared spectrum, it has been determined that |μ(T)(0,eff)| = 85.3(62) mD. A novel approach based on a simple regrouping of angular momentum operators is introduced for decoupling the torsional and rotational degrees of freedom. Copyright 2001 Academic Press.     

Millimeter-Wave Spectroscopy of Kr-CO and Xe-CO Using a Coaxial Jet Spectrometer

A new pulsed supersonic jet millimeter-wave spectrometer with coaxial propagation of the molecular jet and millimeter-waves has been constructed. The coaxial configuration provides greater sensitivity which has been demonstrated by observing b-type transitions of five isotopomers of Kr-(12)C(16)O and seven isotopomers of Xe-(12)C(16)O. These results were analyzed together with a-type transitions obtained by FTMW spectroscopy to determine improved rotational parameters for each isotopomer. The ground vibrational state K = 1 <-- 0 intervals have thus been precisely determined for Kr-CO and Xe-CO. Copyright 2001 Academic Press.     

Direct observation of the ν1 and ν3 fundamental bands of NH2 by difference frequency laser spectroscopy

The ν₁ and ν₃ bands of the NH₂ radical were detected in absorption in the 2.9- to 3.2-μm region using a tunable difference frequency laser and a long-path Zeeman-modulated discharge cell. About 100 rotation-vibration transitions were measured and a simultaneous analysis of the Coriolis-coupled ν₁ and ν₃ states was made. It was found that the ν₁ band is considerably stronger than ν₃, in contrast to the similar molecule H₂O. These results may prove useful in a search for interstellar NH₂ by means of its rotation-vibration spectrum.     

Laser magnetic resonance spectra of NH2 in the 9-μm region: Observation of weak ΔKa = −3 transitions in the ν2 band

Laser magnetic resonance spectra of the ν₂ band of the NH₂ radical have been observed with a CO₂ laser in the 1030–1108-cm^?1 range. The measurements of these weak ΔN = ?1, ΔK_a = ?3 transitions, involving levels with 5 ≤ N ≤ 10, should complement measurements in the higher frequency half of the band (1500–1900 cm^?1) made using CO lasers.