Sensitivity of high-order-harmonic generation to aromaticity

The influence of cyclic electron delocalization associated with aromaticity on the high-order-harmonic generation (HHG) process is investigated in organic molecules. We show that the aromatic molecules benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$ and furan $\left({\mathrm{C}}_4{\mathrm{H}}_4\mathrm{O}\right)$ produce high-order harmonics more efficiently than nonaromatic systems having the same ring structure. We also demonstrate that the relative strength of plateau harmonics is sensitive to the aromaticity in five-membered-ring molecules using furan, pyrrole $\left({\mathrm{C}}_4{\mathrm{H}}_4\mathrm{NH}\right)$, and thiophene $\left({\mathrm{C}}_4{\mathrm{H}}_4\mathrm{S}\right)$. Numerical time-dependent Schrödinger equation simulations of total orientation-averaged strong-field ionization yields show that the HHG from aromatic molecules comes predominantly from the two highest $\pi$ molecular orbitals, which contribute to the aromatic character of the systems.

Figure 1

HHG spectra for (a) benzene (black), cyclohexene (blue), and cyclohexane (red) obtained using 1825 nm light at an intensity of $6.5\times {10}^{13}$ W/${\mathrm{cm}}^2$. (b) For furan (black) and 2,3-dihydrofuran (red) obtained using 1825 nm light at an intensity of $4.5\times {10}^{13}$ W/${\mathrm{cm}}^2$. (c), (d) Cyclohexene-to-benzene signal and 2,3-dihydrofuran-to-furan signal at three different intensities: 4.5 (blue circle), 6.5 (red diamond), and 8.6 (black square) in units of ${10}^{13}$ W/${\mathrm{cm}}^2$.

Figure 2

Top row: Half-cycle ionization probabilities for the lowest eight cation states of benzene, cyclohexene, and cyclohexane at an intensity of $6.5\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. (Note: A small shift of 0.1 eV was added to one component of the degenerate states in benzene and cyclohexane to help make these states distinguishable in the plots.) Bottom row: Half-cycle ionization probabilities for the lowest few cation states of furan and 2,3-dihydrofuran at an intensity of $4.5\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. Probabilities were calculated for a laser wavelength of 1825 nm.

Figure 3

Experimental binding energies for $\pi$ molecular orbitals of pyrrole (left), thiophene (middle), and furan (right) [30]. The Dyson orbitals corresponding to the two highest ionization channels are also shown.

Figure 4

Ratio of harmonic signals. (a) Pyrrole to furan using 1825 nm at three different laser intensities: 2.5 (black circle), 4.5 (blue triangle), and 6.5 (red square) in units of ${10}^{13}$ W/${\mathrm{cm}}^2$. (b) Pyrrole to thiophene, and (c) thiophene to furan using 1825 nm at two different laser intensities: 4.5 (black triangle) and 6.5 (blue square) in units of ${10}^{13}$ W/${\mathrm{cm}}^2$. (d) Thiophene to furan using 1430 nm at two different laser intensities: 4.5 (black diamond) and 6.5 (blue circle) in units of $10{}^{13}$ W/${\mathrm{cm}}^2$.

Figure 5

Ionization probabilities for the calculated lowest six cation doublet states of pyrrole, thiophene, and furan using 1825 nm laser pulses at an intensity of $4.5\times {10}^{13}$ W/${\mathrm{cm}}^2$ over a period of a half laser cycle.