State-to-state rotational energy transfer measurements in silane by infrared double resonance with a tunable diode laser

Abstract

Infrared double resonance spectroscopy has been used to study state-resolved rotational and vibrational energy transfer in vibrationally excited SiH4. Completely specified rotational levels (v,J,Cn) are populated by CO2 laser radiation. Subsequent energy transfer is followed by diode laser transient absorption. The total relaxation efficiencies of the initially populated levels for self-collisions and collisions with Ar and CH4 follow the ordering σ(F2)>σ(A2)>σ(E) and are slightly larger than the Lennard-Jones cross sections. State-to-state rotational energy transfer in the ν4 vibration of SiH4 is extremely state specific. In addition to a differentiation between the A, E, and F symmetry levels, there is a selectivity with respect to the fine-structure levels within each rotational state. A preference for transfer to other levels of the same Coriolis sublevel of ν4 was found. This can be phrased as a Δ(J−R)=0 propensity rule. Principal pathways, only one per J per symmetry, are identified. Within each rotational level, the principal-pathway final states are closely spaced; this effect is related to the clustering of the rovibrational levels of the dyad. Large changes in J are possible in a single collision between silane molecules. A kinetic master equation has been used to model energy flow among rotational levels in silane, from which state-to-state energy transfer parameters could be extracted. Collision-assisted absorption of two CO2 photons into the triad has also been detected. A simple modification of the kinetic analysis allows us to obtain an estimate for the relaxation rate out of the triad levels. These laser pumping and relaxation processes determine the efficiency with which high vibrational levels of silane may be populated by infrared multiple photon excitation.

Reaction of Cl with vibrationally excited CH4 and CHD3: State-to-state differential cross sections and steric effects for the HCl product

“…Our inability to find signal on the R(0) line points to nuclear spin selectivity in our jet, similar to the propensities observed in rotational energy transfer in silane. 52,53 Because Q-branch excitation averages over a number of rotational lines, all of which have similar ␤ axis values, the S fwd ͑2͒ value measured by Q-branch excitation is probably more reliable. Rotational alignment also may have some subtle effect on the steric measurement.…”

Experimental and theoretical study of line mixing in methane spectra. I. The N2-broadened ν3 band at room temperature

“…Although parameters a and b depending on the nuclear spin of the i and j levels are used, 34,51 Eq. ͑B2͒ will fail in modeling line mixing.…”

State-to-State, Rotational Energy-Transfer Dynamics in Crossed Supersonic Jets: A High-Resolution IR Absorption Method

“…Typically, a target molecule in a gas cell is prepared in a single v , J state with a pulsed laser. − Alternatively, a nonthermal distribution of initial states is created via laser photolysis, fast chemical reactions, or collisions with a translationally energetic atom. − As the target molecules approach thermal equilibrium via collisions with a bath gas, the time evolution of the rotational state distribution is monitored with detection techniques such as infrared chemiluminescence, time-resolved Fourier transform spectroscopy, , pulsed laser-induced fluorescence, , or infrared laser absorption spectroscopy. ,, The state-to-state cross sections, averaged over a spread in thermal velocity, can be inferred through detailed kinetic models of the time-dependent populations in each J state. These bath gas relaxation techniques have been used in determinations of state-to-state, rotational energy transfer cross sections for collision systems such as HF with rare gases, CH 4 + CH 4 , ,− CO 2 with translationally hot H atoms, O( 1 D), and electronically excited Br*( 2 P 1/2 ), , self-relaxation in D 2 CO, N 2 , and H 2 and in the open-shell radical systems OH + rare gases, N 2 , and O 2 . By comparison of calculated and experimentally determined rotational energy transfer cross sections and their kinetic energy dependencies, the accuracy of ab initio and empirical potential energy surfaces can be tested for a number of simple atom + molecule collision systems. − ,, …”

“…20 Typically, a target molecule in a gas cell is prepared in a single V, J state with a pulsed laser. [21][22][23][24][25][26][27] Alternatively, a nonthermal distribution of initial states is created via laser photolysis, 28 fast chemical reactions, 29 or collisions with a translationally energetic atom. [30][31][32] As the target molecules approach thermal equilibrium via collisions with a bath gas, the time evolution of the rotational state distribution is monitored with detection techniques such as infrared chemiluminescence, 29 time-resolved Fourier transform spectroscopy, 28,32 pulsed laser-induced fluorescence, 24,33 or infrared laser absorption spectroscopy.…”

“…30,31,34 The state-to-state cross sections, averaged over a spread in thermal velocity, can be inferred through detailed kinetic models of the time-dependent populations in each J state. These bath gas relaxation techniques have been used in determinations of state-to-state, rotational energy transfer cross sections for collision systems such as HF with rare gases, 34 CH 4 + CH 4 , 21,[25][26][27] CO 2 with translationally hot H atoms, O( 1 D), and electronically excited Br*( 2 P 1/2 ), 30,31 selfrelaxation in D 2 CO, 35 N 2 , 36 and H 2 37 and in the open-shell radical systems OH + rare gases, N 2 , and O 2 . 33 By comparison of calculated and experimentally determined rotational energy transfer cross sections and their kinetic energy dependencies, the accuracy of ab initio and empirical potential energy surfaces can be tested for a number of simple atom + molecule collision systems.…”