IR multiphoton absorption and dissociation of molecules by high power CO2 laser

Abstract

The dissociation yield in $CDF_3$ IRMPD at low pressure region (less than 0.4 mbar) decreases with an increase of substrate pressure. However, in the high pressure region (greater than 0.65 mbar), the dissociation yield increases with the increase in substrate pressure and gradually saturates above 27 mbar. The turning point between the opposing pressure effects is found at near 0.4 mbar, which corresponds to a single collision within the laser pulse duration. The rotational hole-filling effects of Ar and $N_2$ are similar up to 13.3 mbar, but $N_2$ exerts greater quenching effects than Ar at higher pressures. In the reaction of $CHF_3/CDF_3/Cl_2$ mixtures, the isotope selective reaction was not observed inspite of the selective excitation of $CDF_3$ at 10R(24) $CO_2$ laser line. However, the effective sensitization of $CHF_3$ through $CDF_3$ excitation made the reaction system be enriched by $CDF_3$. The application of the difference of reaction probability to the isotope enrichment was revealed to be one of the expected candidates in the future. The IRMPA of $C_2H_5Br$ is increased monotonically with the laser fluence. It is suggested that the anharmonicity in pumped vibrational mode plays minor roles on IRMPA of $C_2H_5Br$ system. In the case of IRMPD, below the laser fluence of 100 J/㎠, $C_2H_4$ was detected alone, while $C_2H_2$; $C_2H_6$ and $CH_4$ were observed additionally at higher fluence. The production of $C_2H_6$ attributes to C-Br bond fission, and the formation of $C_2H_2$ was disclosed to the secondary reaction of $C_2H_4^**$ by additional laser photon absorption. $CH_4$ could be produced from $C_2H_4^†$ decomposition. The fluence dependence on the reaction was interpreted in terms of the molecular distribution in highly excited states or model calculation of dissociation probability using EGME. The IRMPD of $C_2H_5Br$ was increased with substrate pressure up to 6.7 mbar due to the large contribution of thermal reaction induced by the collisio...