The population transfer process between the vibrational levels of the HF molecule is investigated by solving the time-dependent Schrödinger equation. The two vibrational levels of υ = 0 and 2 are set to be the initial and target states (an overtone transition). The transition processes under the one- and two-photon resonance conditions are considered, respectively. The influences of the laser intensity, frequency and pulse duration on population transfer are explored. It is found that the one-photon transition process is adiabatic, while the two-photon transition process is diabatic. Other vibrational levels, such as υ = 1 and 3, may participate into the two-photon transition process. Compared to the one-photon transition, a much longer pulse duration is required for the two-photon transition to achieve a high population-transfer efficiency, and the two-photon transition is more sensitive to the laser intensity. Almost a complete population transfer can be achieved for the two-photon resonance case, when the intensity is over 0.0091 a.u. with the appropriate duration (1180 fs) and frequency (0.017674 a.u.).
Measuring two-photon microscopy ultrafast laser pulse duration at the sample plane using time-correlated single-photon counting