State-to-state rate constants for the rotational relaxation of CH(B 2Σ−,v=0,J) in inelastic collisions with Ar

Rotationally inelastic collisions of CN(A(2)Pi) with small molecules

An optical-optical double resonance technique has been utilized to investigate rotational energy transfer of selected rotational/fine-structure levels of CN(A(2)Pi, v = 3) in collisions with the molecular partners CO(2) and CH(4). Total removal rate constants for several rotational/fine-structure levels were measured for both collision partners. State-to-state relative rate constants were determined for several initial levels. These show a strikingly strong collisional propensity to conserve the fine-structure/Lambda-doublet label, akin to our previous observations with the N(2) collision partner. The possible origin of this propensity is discussed.

Vibrational and rotational energy transfers involving the CH B 2Sigma(-) v=1 vibrational level in collisions with Ar, CO, and N2O

With photolysis-probe technique, we have studied vibrational and rotational energy transfers of CH involving the B (2)Sigma(-) (v=1, 0<or=N<or=6, F) state by collisions with Ar, CO, and N(2)O. For the vibrational energy transfer (VET) measurements, the time-resolved fluorescence of the B-X(0,0) band is monitored following the (1,0) band excitation. For the rotational energy transfer (RET) measurements, the laser-induced fluorescence of the initially populated state is dispersed using a step-scan Fourier transform spectrometer. The time-resolved spectra obtained in the nanosecond regime may yield the RET information under a single pressure of the collider. The rate constants of intramolecular energy transfers are evaluated with simulation of kinetic models. The VET lies in the range of 4x10(-12) to 4x10(-11) cm(3) molecule(-1) s(-1), with efficiency following the order of Ar<CO<N(2)O, reflecting the average over Boltzmann rotational distribution. The RET rates are more rapid by one to two orders of magnitude, comparable to the gas kinetic, with the trend of Ar<CO<N(2)O. The transfer rates decrease with increasing N and DeltaN, proceeding via the DeltaN=-1 transitions slightly larger than DeltaN=+1. With the fine-structure labels resolved up to N=6, the fine-structure-conserving collisions prevail increasingly with increasing N in DeltaN not equal 0. The rate constants for the F(2)-->F(1) transitions are larger than the reverse F(1)-->F(2) transitions in DeltaN=0 for the Ar and CO collisions. The trend of fine-structure conservation is along the order of N(2)O<CO approximately Ar. For the CH-Ar collisions, the fine-structure conservation is less pronounced as compared with the v=0 level reported previously. In general, the propensity rules obeyed in the v=0 collision with Ar are valid in v=1, but the latter case shows a weaker tendency. It might be caused by the anisotropy difference of interaction potential when vibrational excitation is considered. For the polyatomic collider, the strong long-range dipole-dipole interaction may have the chance to vary the rotational orientation to increase the fine-structure-changing transitions.

Quantum state-to-state rate constants for the rotationally inelastic collision of CH(B2Sigma(-), nu=0, N-->N') with Ar

We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B (2)Sigma(-), nu=0, N-->N') with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)]. Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and DeltaN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments.

Rotational energy transfer in NO (A 2Σ+,v′=0) by N2 and O2 at room temperature

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Sigma+, v'=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Sigma+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Sigma+ v'=0 state, following excitation by a time-delayed laser at approximately 1.1 microm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.