### Abstract

In this paper we present a new method for studying the collisional relaxation of highly excited molecules in low density gases known as the redistributed successive collisions (RSC) method, and we apply it to the relaxation of CS_{2} by He at 300 K in the vibrational energy range E = 32 640-3180 cm^{-1}. The RSC method involves calculating sequences of collisions, subject to the assumption that rapid vibrational redistribution occurs between each collision. As a result, initial conditions for each trajectory in a sequence are sampled from a microcanonical ensemble that is defined by the final energy and angular momentum of the previous trajectory. The application to He + CS_{2} leads to 〈ΔE〉's that vary linearly with E over the entire energy range considered. The agreement of these 〈ΔE〉's with measured values is good, but there is a qualitative difference in the E dependence of 〈Δ〉 over part of the range of E's. We also examine a second successive collision method that is more appropriate for high-density gases in which the internal coordinates and momenta are conserved (i.e., not redistributed) between collisions (CSC method). We find that a substantial fraction of the CSC ensembles (∼50%) exhibit extremely slow relaxation which in some cases is not complete even after 80 000 collisions. This unphysical result appears to be a classical artifact, and it leads to very small 〈ΔE〉's at medium to low E and a stronger dependence of 〈Δ〉 on E (close to quadratic) at high E. Omission of these slowly relaxing ensembles from the CSC ensemble average leads to CSC 〈ΔE〉's which are nearly identical to those from the RSC calculation. An analysis of the distribution of energy among vibrational modes in the CSC calculations indicates that the slow relaxation arises from energy becoming frozen in the asymmetric stretch of CS_{2}. The influence of the CS_{2} intramolecular dynamics on the collisional relaxation is considered, and we find evidence of abrupt collision induced intramolecular energy redistribution due to nonlinear resonance formation.

Original language | English |
---|---|

Pages (from-to) | 770-779 |

Number of pages | 10 |

Journal | Journal of Chemical Physics |

Volume | 89 |

Issue number | 2 |

Publication status | Published - 1988 |

### Fingerprint

### ASJC Scopus subject areas

- Atomic and Molecular Physics, and Optics

### Cite this

**Theoretical studies of collisional energy transfer in highly excited molecules : The importance of intramolecular vibrational redistribution in successive collision modeling of He+CS _{2}.** / Bruehl, Margaret; Schatz, George C.

Research output: Contribution to journal › Article

}

TY - JOUR

T1 - Theoretical studies of collisional energy transfer in highly excited molecules

T2 - The importance of intramolecular vibrational redistribution in successive collision modeling of He+CS2

AU - Bruehl, Margaret

AU - Schatz, George C

PY - 1988

Y1 - 1988

N2 - In this paper we present a new method for studying the collisional relaxation of highly excited molecules in low density gases known as the redistributed successive collisions (RSC) method, and we apply it to the relaxation of CS2 by He at 300 K in the vibrational energy range E = 32 640-3180 cm-1. The RSC method involves calculating sequences of collisions, subject to the assumption that rapid vibrational redistribution occurs between each collision. As a result, initial conditions for each trajectory in a sequence are sampled from a microcanonical ensemble that is defined by the final energy and angular momentum of the previous trajectory. The application to He + CS2 leads to 〈ΔE〉's that vary linearly with E over the entire energy range considered. The agreement of these 〈ΔE〉's with measured values is good, but there is a qualitative difference in the E dependence of 〈Δ〉 over part of the range of E's. We also examine a second successive collision method that is more appropriate for high-density gases in which the internal coordinates and momenta are conserved (i.e., not redistributed) between collisions (CSC method). We find that a substantial fraction of the CSC ensembles (∼50%) exhibit extremely slow relaxation which in some cases is not complete even after 80 000 collisions. This unphysical result appears to be a classical artifact, and it leads to very small 〈ΔE〉's at medium to low E and a stronger dependence of 〈Δ〉 on E (close to quadratic) at high E. Omission of these slowly relaxing ensembles from the CSC ensemble average leads to CSC 〈ΔE〉's which are nearly identical to those from the RSC calculation. An analysis of the distribution of energy among vibrational modes in the CSC calculations indicates that the slow relaxation arises from energy becoming frozen in the asymmetric stretch of CS2. The influence of the CS2 intramolecular dynamics on the collisional relaxation is considered, and we find evidence of abrupt collision induced intramolecular energy redistribution due to nonlinear resonance formation.

AB - In this paper we present a new method for studying the collisional relaxation of highly excited molecules in low density gases known as the redistributed successive collisions (RSC) method, and we apply it to the relaxation of CS2 by He at 300 K in the vibrational energy range E = 32 640-3180 cm-1. The RSC method involves calculating sequences of collisions, subject to the assumption that rapid vibrational redistribution occurs between each collision. As a result, initial conditions for each trajectory in a sequence are sampled from a microcanonical ensemble that is defined by the final energy and angular momentum of the previous trajectory. The application to He + CS2 leads to 〈ΔE〉's that vary linearly with E over the entire energy range considered. The agreement of these 〈ΔE〉's with measured values is good, but there is a qualitative difference in the E dependence of 〈Δ〉 over part of the range of E's. We also examine a second successive collision method that is more appropriate for high-density gases in which the internal coordinates and momenta are conserved (i.e., not redistributed) between collisions (CSC method). We find that a substantial fraction of the CSC ensembles (∼50%) exhibit extremely slow relaxation which in some cases is not complete even after 80 000 collisions. This unphysical result appears to be a classical artifact, and it leads to very small 〈ΔE〉's at medium to low E and a stronger dependence of 〈Δ〉 on E (close to quadratic) at high E. Omission of these slowly relaxing ensembles from the CSC ensemble average leads to CSC 〈ΔE〉's which are nearly identical to those from the RSC calculation. An analysis of the distribution of energy among vibrational modes in the CSC calculations indicates that the slow relaxation arises from energy becoming frozen in the asymmetric stretch of CS2. The influence of the CS2 intramolecular dynamics on the collisional relaxation is considered, and we find evidence of abrupt collision induced intramolecular energy redistribution due to nonlinear resonance formation.

UR - http://www.scopus.com/inward/record.url?scp=36549103232&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=36549103232&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:36549103232

VL - 89

SP - 770

EP - 779

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 2

ER -