Resonancelike enhancement in high-order above-threshold ionization of polyatomic molecules

We investigate the resonance-like enhancement (RLE) in high-order above-threshold ionization (ATI) spectra of the polyatomic molecules ${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_6$. In the spectrum-intensity maps, strong and weak RLE areas emerge alternatively for both ${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_6$ but in different sequences. Theoretical calculations using the strong-field approximation reproduce the experimental observation and analysis shows that the different characteristics of the two molecules can be attributed to interference effects of molecular orbitals with different symmetries. For ${\mathrm{C}}_2{\mathrm{H}}_4$, the RLE structures are attributed to C–C centers of the highest occupied molecular orbital (HOMO) orbital. For ${\mathrm{C}}_2{\mathrm{H}}_6$, in contrast, the C–C centers of the HOMO and HOMO-1 orbitals do not contribute to the RLE due to destructive interference but the hydrogen centers of the bonding HOMO-1 orbital give rise to the multiple RLE regions. In addition, clear experimental evidence of the existence of two types of the RLE and their dependence on the parity of ground state is shown. Our result, which strongly supports the channel-closing mechanism of the RLE, for the first time reveals the important role of low-lying orbitals and the differing roles of different atomic centers in the high-order ATI spectrum of molecules.

Figure 1

The measured ATI spectra of ${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_6$ covering a wide intensity range. The horizontal axis reflects the laser intensity and the vertical axis gives the kinetic energy of ATI electrons. The false-color scale reflects the ionization yields. The bright yellow horizontal lines indicate the RLE structures (marked by arrows), as discussed in the text.

Figure 2

The experimental electron kinetic energy spectra of ${\mathrm{C}}_2{\mathrm{H}}_4$ [panels (a) and (b)] and ${\mathrm{C}}_2{\mathrm{H}}_6$ [panels (c) and (d)]. Only the plateau regions are shown for comparison of different types of RLE. The numbers given in the legends are peak laser intensities with units of TW/${\mathrm{cm}}^2$.

Figure 3

The simulated ATI spectra of ${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_6$.

Figure 4

The simulated electron kinetic energy spectra of ${\mathrm{C}}_2{\mathrm{H}}_4$ [panels (a) and (b)] and ${\mathrm{C}}_2{\mathrm{H}}_6$ [panels (c) and (d)].

Figure 5

The simulated ATI spectra corresponding to different occupied orbitals of ${\mathrm{C}}_2{\mathrm{H}}_6$: ${\mathrm{HOMO}}_1$ in the first row and HOMO-1 in the second row. The spectra corresponding to C and H components in each orbital are also shown in the second and third columns, respectively.