Pulse-shape-dependent strong-field ionization viewed with velocity-map imaging

We explore strong field molecular ionization with velocity map imaging of fragment ions produced by dissociation following ionization. Our measurements and ab initio electronic structure calculations allow us to identify various electronic states of the molecular cation populated during ionization, with multiple pathways to individual states highlighted by the pulse shape dependence. In addition, we show that relative populations can be reconstructed from our measurements. The results illustrate how strong field molecular ionization can be complicated by the presence and interaction of multiple cationic states during ionization.

Figure 1

These graphs summarize the results from the calculations described in Sec. 3. Panel (a) (based on SA-CASSCF computations) shows the energy of the five lowest adiabatic states of CH${}_2$BrI${}^{+}$ along the bending normal mode coordinate $u$. The orange arrows indicate several FC points, FC${}_1^{\prime }$ for the vertical ionization from the neutral ground state, FC${}_2$ for the close lying vertical $V_1$/$V_3$, $V_1$/$V_4$ resonances, and FC${}_3$ for the three photon $V_1$/$V_5$ resonance. (The exact positions of the resonances are indicated by the black arrows). Panel (b) (based on MRCI combined with experimental values) shows the minimum electronic energy of the first few ionic states (left column) and the minimum electronic energy required to create CH${}_2$Br${}^{+}$ in various electronic states: A, B, C, D, and E.

Figure 2

2D velocity distribution of CH${}_2$Br${}^{+}$ for a single unshaped laser pulse. The color axis represents the ion yield on a logarithmic scale. The $x$ and $y$ axis show the velocity along the $x$ and $y$ direction of the detected ions. The laser polarization was along the $x$ axis. The sample was prepared at room temperature and was not oriented. One can clearly see that the total ion yield can be sorted into a slow and fast group of ions. Also, the fast group exhibits a strong tendency to be ejected along the laser polarization. The dashed black lines indicate the borders between the parallel (P) and the orthogonal sectors (O).

Figure 3

This graph shows two typical KER distributions of CH${}_2$Br${}^{+}$ fragments. The solid blue (black) curve was taken at low intensity ($2.0\times {10}^{13}$ W cm${}^{-2}$), while the dotted red (gray) curve corresponds to high laser intensity ($3.9\times {10}^{13}$ W cm${}^{-2}$). The solid green (light gray) curve shows the asymmetry parameter vs KER for the dotted red curve. The total KER was calculated based on the measured CH${}_2$Br${}^{+}$ kinetic energy, assuming a two-body dissociation process.

Figure 4

CH${}_2$Br${}^{+}$ yield vs KER and pump-probe time delay $Y(t,E)$ summed over both sectors.

Figure 5

Ion yields as a function of pump-probe delay. The total CH${}_2$Br${}^{+}$ and parent ion yields are shown in light blue (upper light gray line) and dark blue (lower dark gray line), respectively. With detailed knowledge of the reconstructed indirect contributions (shown in Fig. 7) the time behavior of the indirect populations on $V_3$ and $V_4$ (leading to CH${}_2$Br${}^{+}$) can be enhanced. The green (lower gray) curve shows the CH${}_2$Br${}^{+}$ signal integrated from 0 to 400 meV. The red (upper dark gray) curve shows the asymmetric part of the total ion yield for a specific KER interval: {${\int }_{400\text{meV}}^{1.1\text{eV}}dE[Y_p(E,t)-Y_o(E,t)]$}.

Figure 6

CH${}_2$Br${}^{+}$ yield as a function of KER for the Fourier component of the pump-probe data at 95 cm${}^{-1}$. This data is the one-dimensional Fourier transform of the data shown in Fig. 4 evaluated at 95 cm${}^{-1}$: ${\tilde{Y}}_{o,p}(\omega =95$ cm${}^{-1},E)$. Both sectors (O and P) are analyzed separately. The Fourier transformed signal ${\tilde{Y}}_{o,p}(95$ cm${}^{-1}$ $,E)$ is split into its absolute value (solid lines) and its phase (dashed lines).

Figure 7

Reconstructed KER distributions for the indirect $V_{3,4}$ [red (dark gray)] and indirect $V_5$ [green (light gray)] population, both sectors are shown individually. The blue (black) line shows the KER distribution for a single unshaped pulse for comparison.

Figure 8

KER distributions for CH${}_2$Br${}^{+}$ (showing the slow peak only). The distributions are shown for the orthogonal and the parallel sectors separately. The graph compares the distributions for excitation with an unshaped pulse vs a strongly chirped pulse. All graphs have been normalized to the maximum ion yield.

Figure 9

Dependence of KER distribution of the slow peak on chirp. The dashed graph shows the calculated ratio “indirect $V_{3,4}$” to “direct $V_{3,4}$” population vs chirp. The solid graph shows the ratio extracted from our measurements by using the reconstructed direct and indirect $V_{3,4}$ and indirect $V_5$ distributions as a basis set.