vuv fluorescence from selective high-order multiphoton excitation of N2 -: art. no. 053407

Recent fluorescence studies suggest that ultrashort pulse laser excitation may be highly selective. Selective high-intensity laser excitation holds important consequences for the physics of multiphoton processes. To establish the extent of this selectivity, we performed a detailed comparative study of the vacuum ultraviolet fluorescence resulting from the interaction of N-2 and Ar with high-intensity infrared ultrashort laser pulses. Both N-2 and Ar reveal two classes of transitions, inner-valence ns<--np and Rydberg np<--n'l'. From their pressure dependence, we associate each transition with either plasma or direct laser excitation. Furthermore, we qualitatively confirm such associations with the time dependence of the fluorescence signal. Remarkably, only N-2 presents evidence of direct laser excitation. This direct excitation produces ionic nitrogen fragments with inner-valence (2s) holes, two unidentified transitions, and one molecular transition, the N-2(+):X (2)Sigma(g)(+)<--C (2)Sigma(u)(+). We discuss these results in the light of a recently proposed model for multiphoton excitation.