High-order-harmonic generation from molecular isomers with midinfrared intense laser pulses

We present theoretical calculations of high-order-harmonic generation (HHG) from stereoisomers of 1,2-dichloroethylene (C${}_2$H${}_2$Cl${}_2$) and 2-butene (C${}_4$H${}_8$) based on the quantitative rescattering theory. Our results show that the HHG spectra from these cis and trans isomers with intense midinfrared laser pulses are clearly distinguishable, even when the molecules are randomly oriented, in good agreement with the recent experiments by Wong et al. [Phys. Rev. A 84, 051403(R) (2011)]. We found that the angle-averaged tunneling ionization yields and photoionization cross sections from the cis and trans isomers for both molecules are nearly identical. The origin of the differences in HHG spectra is traced as due to the interplay in the angular dependence of the photoionization (or photorecombination) cross section and ionization rate.

Figure 1

Total (integrated) photoionization cross section from the HOMO of cis-DCE and trans-DCE. The insets show the HOMOs and molecular frame coordinates used in our calculations. All atoms are in the $YZ$ plane for cis-DCE and the $XY$ plane for trans-DCE.

Figure 2

(a), (b) Ionization rate from MO-ADK for cis-DCE and trans-DCE, respectively, at an intensity of $0.6\times {10}^{14}$ W/cm${}^2$. (c), (d) Ionization probability (scaled by a factor of 100) from the SFA for the HOMO of cis-DCE and trans-DCE, respectively. Only the electron emission along the laser polarization is included during a half-cycle laser pulse of a 1800 nm wavelength and an intensity of $0.6\times {10}^{14}$ W/cm${}^2$. (e), (f) Transition dipole amplitude for cis-DCE and trans-DCE, respectively, at a photon energy of 39 eV.

Figure 3

(a) HHG spectra (only the envelopes are shown) from cis- and trans-DCE for a 1800 nm wavelength laser and an intensity of $0.6\times {10}^{14}$ W/cm${}^2$. The calculations were carried out with the QRS using the SFA ionization rate. (b) cis-DCE to trans-DCE intensity ratio for different laser wavelengths of 1300, 1500, and 1800 nm. The laser intensity is fixed at $0.6\times {10}^{14}$ W/cm${}^2$. (c) Induced dipole amplitude (weighted by $\sin \theta$, vertical axis) and phase (color code) for H65 (photon energy of 45 eV) from cis-DCE. The phase is given in units of $\pi$. (d) Same as (c) but for trans-DCE. The laser parameters are the same as for (a).

Figure 4

(a) HHG spectra (only the envelopes are shown) from cis and trans isomers of C${}_4$H${}_8$ for a 1700 nm wavelength laser with an intensity of $0.4\times {10}^{14}$ W/cm${}^2$, and a pulse duration of 40 fs. The calculations were carried out with the QRS using the SFA ionization rate. (b) cis to trans intensity ratio for different laser wavelengths of 1300, 1500, and 1700 nm. The laser intensity is fixed at $0.6\times {10}^{14}$ W/cm${}^2$. The QRS result with the use of the MO-ADK rate is also shown for the case of 1700 nm.