Rotational effects in multiphoton excitation of hot molecules

The visible chemiluminescence resulting from infrared multiphoton excitation (MPE) of CrO₂Cl₂ is used to study the effect of inert gas addition on the yield and branching ratio of MPE induced reactions. In some cases, addition of foreign gases increases the chemiluminescence yield, and also leads to a blue shift in the emission spectrum. This result may indicate bottleneck formation in the quasicontinuum region. This bottlenecking is discussed using a model that calls for a gradual broadening of single rotational lines as energy is accumulated in the molecule. Under appropriate conditions, fast rotational relaxation can lead to higher average vibrational energy, as well as to a significant population increase in the high energy tail of the vibrational distribution. A stochastic method is used to solve the rate equations. This computationally simple technique is particularly useful in the limiting case of fast rotational relaxation.