Infrared Multiphoton Excitation of Small Polyatomic Molecules.

A systematic study of the collisionless infrared multiphoton excitation (IRMPE) and dissociation (IRMPD) of three and four atom molecules is presented. Larger polyatomic molecules have been studied in the past but this is the first detailed study of IRMPE in small molecules. Unlike larger polyatomics, there is a strong laser intensity, as well as fluence, dependence in the excitation of small polyatomics. Resonantly enhanced multiphoton excitation, rather than stepwise incoherent excitation, provides the pumping mechanism. Specifically, collisionless IRMPE and IRMPD of a molecule with only three vibrational modes, SO(,2), is shown for the first time. Experiments on OCS show that, like other small molecules, very high laser intensity is necessary for IRMPE. This is at variance with previously published results. A four atom molecule, NH(,3), can undergo IRMPD, but shows the effects of the large anharmonicity of the pumped vibrational transition. Another four atom molecule, DN(,3), undergoes interesting chemistry following IRMPD. The dissociation fraction for each of the small molecules is only on the order of 10('-3) at laser intensities of 100 GW/cm('2) and fluences of 100 J/cm('2). Though NO(,2) at low vibrational energy does not undergo IRMPE under CO(,2) laser excitation, it will undergo IRMPE if previously excited to high vibrational levels by a visible laser. Near the dissociation threshold strictly fluence dependent excitation is possible for short, < 1 ns, CO(,2) laser pulses. Longer pulses exhibit intensity effects. The importance of the breakdown of rotational selection rules to the IRMPE of highly excited NO(,2) is shown. These experimental results are all in agreement with the well developed three region Quasicontinuum Model which describes IRMPE in larger polyatomics. The strong intensity effects exhibited in three and four atom molecules are due to the sparse density of accessible states for excitation. Near resonant states participate in high intensity excitation as intermediates that enhance the probability of true multiphoton transitions, but they cannot be populated at the end of the laser pulse. IRMPE in small molecules generally becomes more efficient as the molecule absorbs energy and the density of states grows.