Imaging Electronic Excitation of NO by Ultrafast Laser Tunneling Ionization

Tunneling-ionization imaging of photoexcitation of NO has been demonstrated by using few-cycle near-infrared intense laser pulses (8 fs, 800 nm, $1.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$). The ion image of ${\mathrm{N}}^{+}$ fragment ions produced by dissociative ionization of NO in the ground state, NO $(X^2\mathrm{\Pi },2\pi )\to {\mathrm{NO}}^{+}+{\mathit{e}}^{-}\to {\mathrm{N}}^{+}+\mathrm{O}+{\mathit{e}}^{-}$, exhibits a characteristic momentum distribution peaked at 45° with respect to the laser polarization direction. On the other hand, a broad distribution centered at $\sim 0°$ appears when the $A^2{\mathrm{\Sigma }}^{+}$ ($3s\sigma$) excited state is prepared as the initial state by deep-UV photoexcitation. The observed angular distributions are in good agreement with the corresponding theoretical tunneling ionization yields, showing that the fragment anisotropy reflects changes of the highest-occupied molecular orbital by photoexcitation.

Figure 1

(a) The highest-occupied molecular orbitals, $2\pi$ and $3s\sigma$, in the $X^2\mathrm{\Pi }$ and $A^2{\mathrm{\Sigma }}^{+}$ state. (b) Dissociative ionization of NO in intense laser fields by tunneling ionization and electron rescattering. Potential energy curves are adopted from Refs. [18, 19, 20]. The Franck-Condon region for the $X^2\mathrm{\Pi }$ state, and that for the $A^2{\mathrm{\Sigma }}^{+}$ state prepared by DUV excitation (226 nm), are shown with red and blue bars, respectively. The horizontal arrows indicate the energies of the $c^3\mathrm{\Pi }$ state (orange) and the $B^1\mathrm{\Pi }$ state (green) populated by the corresponding Franck-Condon transitions. The inset shows the definition of the molecular-axis angle $\beta$, where an arrow with $ϵ$ represents the laser polarization direction.

Figure 2

Momentum images of the ${\mathrm{N}}^{+}$ fragment ions produced by DI starting from (a) the $X^2\mathrm{\Pi }$ state and (b) the $A^2{\mathrm{\Sigma }}^{+}$ state in few-cycle intense laser fields (8 fs, $1.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$). Symmetries with respect to the $x$ and $y$ axis are utilized to reduce the statistical uncertainty. The probe NIR laser polarization direction is denoted with $ϵ$. (c),(d) Polar plots of the fragment angular distributions obtained for the $X^2\mathrm{\Pi }$ and $A^2{\mathrm{\Sigma }}^{+}$ initial states, respectively. The distributions are evaluated on the $c^3\mathrm{\Pi }$ dissociation pathway (see text). Solid lines are theoretical tunneling-ionization yields calculated by WFAT under the adiabatic approximation.

Figure 3

The total KER spectra obtained with ($A$) pump off, ($B$) pump on, ($C$) pump only, and ($D$) the net signal, where spectra ($A$) and ($D$) correspond to the spectra of the dissociative ionization from the $X^2\mathrm{\Pi }$ and $A^2{\mathrm{\Sigma }}^{+}$ states, respectively. The arrows indicate peaks assigned to the $c^3\mathrm{\Pi }$ and $B^1\mathrm{\Pi }$ dissociation pathways [see Fig. 1].

Figure 4

Relative yields of ${\mathrm{N}}^{+}$ fragment ions as a function of the angle $\theta$ of a quarter-wave plate with the $X^2\mathrm{\Pi }$ (red circles) and $A^2{\mathrm{\Sigma }}^{+}$ (blue squares) states of NO as the initial state. The solid line is a result of the least-squares fitting to the experimental data for the ground state, while the dashed line represents a theoretical prediction for the excited state (see text for details).