Photodissociation spectroscopy via a rovibrational resonance in intense UV pulses

Photodissociation dynamics via a rovibrational resonance in intense UV pulses is investigated theoretically, using a showcase ${\text{CH}}^{+}$ molecule promoted to the $\text{C}{}^1\mathrm{\Sigma }^{+}$ valence excited electronic state with a potential barrier, thus giving access to study shape resonance controlled by the pulse frequency. Simulations of the kinetic energy release (KER) and angular distribution of the photofragments (ADP) spectra show dramatic differences for the cases when the pulse is tuned on and off the rovibrational resonance. It shows that as the increasing pulse intensity for the transitions to shape resonance, the KER spectra develop into new and higher energy peaks overlying on the broadened background, which is explained by the involvement of other resonances with higher partial waves through electronic Rabi flopping between the ground and excited electronic states. Those nonlinear contribution increases drastically the probability of photofragmentation along the laser polarization in the ADP spectra. The coincident KER-ADP spectra reveal clearly the correlated dynamics in the vicinity of the dissociation barrier. The present work opens possibilities for the manipulation of ultrafast photodissociation dynamics with the help of resonance states in the continuum.

Figure 1

(a) Potential energy curves (PECs) of ${\text{CH}}^{+}$ for the ground $\mathrm{X}{}^1\mathrm{\Sigma }^{+}$ and first valence excited $\text{C}{}^1\mathrm{\Sigma }^{+}$ electronic states; (b) the permanent dipole moments (PDM) and the transition dipole moments (TDM) for the two electronic states (data are reproduced from Ref. [46]). The strong UV pulse with $\omega =8.25$ eV is tuned into the resonance with $\nu =2$ vibrational level leading to direct fragmentation, and another case with $\omega =8.53$ eV above the potential barrier is studied for comparison.

Figure 2

(a), (b) KER spectra and (c), (d) ADP of the dissociation fragments from the $\mathrm{C}{}^1\mathrm{\Sigma }^{+}$ state with the UV frequencies $\omega =8.25$ eV [(a), (c)] and $\omega =8.53$ eV [(b), (d)], respectively. The calculations are performed for six UV pulse intensities shown by different line colors and symbols [see the legends in (a)].

Figure 3

Coincident KER-ADP spectra for (a), (c), (e) resonance excitation $\omega =8.25$ eV and (b), (d), (f) nonresonance excitation $\omega =8.53$ eV, with three pulse intensities ${2\times {10}^{13}\text{W/cm}}^2$ [(a), (b)], ${6\times {10}^{13}\text{W/cm}}^2$ [(c), (d)], and ${1\times {10}^{14}\text{W/cm}}^2$ [(e), (f)]. The dotted lines from bottom to top in (a), (c), (e) corresponding to the energy levels of rovibrational resonances pertaining to states $J=1,3,$ and 5, respectively; the dotted lines in (b), (d), (f) indicate the energy of direct stimulating.

Figure 4

Coincidence KER-ADP spectra for (a) resonance excitation $\omega =8.25$ eV and (b) nonresonance excitation $\omega =8.53$ eV with the UV pulse intensity ${I=8\times {10}^{13}\text{W/cm}}^2$. Correspondent ADP profiles [(c), (d)] for several fixed KER shown in (a) and (b), solid lines and dotted lines are the ADP obtained by coincidence spectra using using Eq. (2) and reconstructed ones using Eq. (4), respectively.