Theory of laser cooling of polyatomic molecules in an electronically excited state

A detailed theoretical study is presented for the vibrational population di stribution of polyatomic molecules which results from electronic excitation from a thermal ground state. If the vibrational frequencies of the excited state are lower than the ground-state frequencies and if position shifts a re not too large, then there exist excitation frequencies for which the exc ited-state vibrational distribution will be cooled in comparison to the gro und state. An analytic theory for the vibrational distribution in the excit ed state is obtained by noting that the fast dephasing of a polyatomic mole cule after excitation allows for the development of a Gaussian approximatio n for the excitation process. We show that the equilibrium energy distribut ion of a polyatomic molecule as well as the nascent distribution after exci tation are well approximated as Gaussian. The average energy in the excited state is then found to be a quadratic function of the excitation frequency . If cooling takes place, it will usually be maximal for an excitation freq uency which is to the red of the ground electronic state to ground electron ic state excitation frequency. Cooling is not necessarily a quantum effect, it may also be found in the classical limit, in which one ignores quantiza tion of the vibrational levels. The generality of the Gaussian approximatio n opens the way for theoretical treatment of anharmonic polyatomic molecule s, using quantum Monte Carlo techniques. (C) 1999 American Institute of Phy sics.