Strong Field Molecular Ionization in the Impulsive Limit: Freezing Vibrations with Short Pulses

We study strong-field molecular ionization as a function of pulse duration. Experimental measurements of the photoelectron yield for a number of molecules reveal competition between different ionization continua (cationic states) which depends strongly on pulse duration. Surprisingly, in the limit of short pulse duration, we find that a single ionic continuum dominates the yield, whereas multiple continua are produced for longer pulses. Using calculations which take vibrational dynamics into account, we interpret our results in terms of nuclear motion and nonadiabatic dynamics during the ionization process.

Figure 1

Photoelectron spectra [yield vs photoelectron kinetic energy (KE)] for ionization of ${\mathrm{CH}}_2\mathrm{IBr}$ with different pulse durations.

Figure 2

Ratio of $D_1/D_0$ as a function of pulse duration $\tau$ for three different molecules.

Figure 3

Top panel: Optical spectra for broadband pulse (solid black line) and the result of summing narrow-band spectra (dashed red line). Bottom panel: photoelectron spectra of ${\mathrm{CH}}_2\mathrm{IBr}$ acquired with full bandwidth optical spectrum (solid black line), and the result of forming a linear combination of photoelectron spectra each acquired with narrow-band optical excitation (dashed red line). The latter are added in proportion to their spectral weights as shown in the inset and described in the text.

Figure 4

Left panel: calculated potential energy surfaces for ${\mathrm{CH}}_2\mathrm{IBr}$ along the ${\mathrm{CH}}_2$ wagging mode (u). FC: Franck-Condon point for excitation from the minimum of $S_0$ ($u=0$). Upper right panel: calculated photoelectron spectra for 40 fs pulse. Middle right panel: calculated photoelectron spectra for 10 fs pulse. Bottom right panel: calculated $D_1/D_0$ yield ratio as a function of pulse duration $\tau$.