Deactivation and reaction of excited states of Rb in collisions with H2, CH4 and C2H6

Alkali vapor lasers commonly use methane (CH4) or ethane (C2H6) to induce energy transfer between the optically pumped level (n2P3/2) and the upper laser level (n2P1/2). A complication is that the alkali metal eventually reacts with the hydrocarbons. The reaction becomes exothermic for alkali atoms that have been excited by energy pooling processes such as Rb(5p)+Rb(5p)→Rb(*)+Rb(5s), where * indicates 6s, 6p or 4d. We have used laser pump-probe methods to examine the reactions of Rb(n2P) with CH4, and C2H6 for states with n=6 and 7. Pump-probe measurements indicated a loss of Rb due to a reaction. Surprisingly, the RbH product was not detected with CH4, and C2H6. High-level ab initio calculations have been used to study the reactive interactions between Rb, methane and ethane.