Ionization and Fragmentation of Alkylphenols by 0.8−1.5 μm Femtosecond Laser Pulses

Abstract

Ionization and fragmentation were studied on alkylphenols with long alkyl chains (p-(C6H4)(OH)(C(n)H(2n+1)), n = 1,3,5,8,9) and, for reference, on alkylbenzenes ((C6H5)(C(n)H(2n+1)), n = 1,3,5,7,9) by intense femtosecond laser pulses, typically with 43 fs duration at 0.8 microm and 140 fs at 1.3 microm in an intensity range of 10(14) W cm(-2). The major products were the corresponding molecular and C7 fragment ions from the alkylphenols and alkylbenzenes. The molecular ion yields decreased from nearly 1 (n = 1) to 0.3-0.5 (n = 9) when the carbon number in the alkyl chain increased for both excitation wavelengths. Higher yields of the molecular ions were observed at a longer wavelength of 1.3 microm. The long wavelengths in the range of 1.3-1.5 microm were used to determine whether or not -OH absorption had any increase in fragment ions. No effect was observed by vibrational overtone excitation of the -OH group in this wavelength range. Direct dissociation by cation absorption is the most plausible explanation of the present fragmentation results. Other possible mechanisms were discussed, including a statistical model, an effect of electron rescattering, a multiactive electron model, and dissociation from the superexcited state. In the case of cyclohexane, nonresonant wavelength excitation with a pulse of 1.3 microm (150 fs) effectively suppressed fragmentation more than excitation by a resonant but short-duration pulse (0.8 microm, 15 fs).