Zhen-Dong Sun

Separation and conversion dynamics of four nuclear spin isomers of ethylene.

Molecules with three or more nuclei of nonzero spin exist as discrete spin isomers whose interconversion in the gas phase is generally considered improbable. We have studied the interconversion process in ethylene by creating a sample depleted in the B2u nuclear spin isomer. The separation was achieved through spatial drift of this isomer induced by resonant absorption of narrow-band infrared light. Evolution of the depleted sample revealed conversion between B2u and B3u isomers at a rate linearly proportional to pressure, with a rate constant of 5.5 (±0.8) × 10–4 s–1 torr–1. However, almost no change was observed in the Ag isomer populations. The results suggest a spin conversion mechanism in C2H4 via quantum relaxation within the same inversion symmetry.

Infrared–microwave double-resonance spectroscopy of CH 3 OH by use of sidebands of CO 2 laser lines

An infrared–microwave double-resonance technique using microwave sidebands of CO2 laser lines as an infrared source has been applied for observation of rotational lines of the methanol molecule. Frequencies of more than 50 rotational lines in the excited C—O stretching vibrational state (vco=1) have been measured with good precision and have been compared with those reported in infrared studies. Many of them agree within several megahertz, although some lines show differences of >10 MHz. The pressure dependence of the double-resonance signals for two low-J microwave transitions belonging to the ground and the vco=1 states, respectively, have been observed for sample pressures as high as 0.4 Torr. For the former transition the signal has been observed to change its sign at higher pressures. Rate equation analysis explains the observed pressure dependence quantitatively and allows us to understand the physical processes involved in the double resonance.

Sub-Doppler Spectroscopy by Use of Microwave Sidebands of CO2 Laser Lines Applied to the C-O Stretching Fundamental Band of Methanol

Microwave sidebands of CO2 laser lines were used as an infrared source in infrared microwave double resonance spectroscopy and infrared saturation spectroscopy to study the C-O stretching fundamental band of methanol. In the former application, rotational lines in the vCO = 1 state have been observed with good signal to noise ratios and physical processes involved in this method have been discussed. In the latter method, (vt, A/E, K) = (2, A, 4) sequence transitions in the C-O stretching fundamental band have been identified and term values for the (2, A, 4) levels in the vCO = 1 state have been determined.

Nuclear quadrupole coupling operator for a molecule with an internal rotor

The sequential contact transformation technique is applied to the nuclear quadrupole coupling operator for a molecule containing a threefold symmetric internal rotor in order to be consistent with the transformed vibration–torsion–rotation Hamiltonian to be used in the analysis of the spectra of the molecule. Various higher‐order effects on the nuclear quadrupole coupling energies, such as the centrifugal distortion, the anharmonic corrections and so on, are formulated for a molecule containing one quadrupolar nucleus. The detailed discussion is presented of vibrational–torsional–rotational dependence of these higher corrections to the quadrupole hyperfine energies.

Centrifugal distortion effects for a molecule with two internal rotors

The centrifugal distortion effects for a molecule containing two symmetric internal rotors are formulated. The explicit expressions that relate the distortion constants to fundamental molecular parameters are given, which are applicable for analysing molecular structure from the parameters determined from a fit-to-observed spectrum or for calculating the centrifugal distortion constants from well known fundamental molecular parameters. The reduced torsion - rotation Hamiltonian is given for a molecule with two internal rotors with a symmetry plane which contains both internal rotation axes.

Vibration-torsion-rotation polarizability tensor operator for a molecule with an internal rotor

The sequential contact transformation technique is applied to the vibration-torsion-rotation polarizability tensor operator for a molecule containing a threefold symmetric internal rotor. Detailed discussions about various types of polarizability tensor operators and corresponding vibration-torsion-rotation transitions are presented. The formalism of the transformed polarizability tensor operator makes it possible to analyze various high-order vibration-torsion-rotation interactions and their effects as well as to determine the intensities of transitions between vibration-torsion-rotation states within the Raman spectrum of the molecule. As an example, the transition matrix elements of the transformed polarizability tensor operators are discussed for CH3OH-like molecules.

Derivation of the torsion - rotation Hamiltonian for a molecule with two internal rotors

A second-order quantum-mechanical torsion - rotation Hamiltonian for a molecule with two rotors has been derived by using a Taylor and Fourier series expansion on the effective total potential energy of the molecular system and on the element of inverse moment of inertia tensor in the original Hamiltonian. The effects of interactions between rotations and internal rotations, between the two rotors and also the effects of vibrations on the internal rotation barrier in a molecule with two rotors are formulated.