Peculiarities of molecule photodissociation under influence of ultrashort electromagnetic pulses: Nonlinear dependence of probability on pulse duration

The paper considers novel behavior of molecule photodissociation probability under the influence of electro-magnetic pulses in the femtosecond range. The study considers as examples the photodissociation of methane, ethylene and water molecules. The probability of photodissociation under radiation is calculated as a function of pulse duration for different values of the carrier frequency. In the limit of long pulses, the process probability, as expected, increases linearly with pulse duration. However, the dependence of photodissociation probability on pulse duration for the pulses in the femto- and sub-femtosecond ranges exhibits strong nonlinearity. The non-monotonic effects occur at carrier pulse frequencies corresponding to the minimum values of the photo-dissociation cross-section. It is remarkable that the observed nonlinearity is described in the frame of the first order of perturbation theory for relatively small field amplitude and unrelated to the strong field effects. The proposed method of describing photodissociation is quite general and suitable for studying such processes under the influence of electromagnetic pulses of any duration.