Observation of laser-induced electronic structure in oriented polyatomic molecules

Abstract

All attosecond time-resolved measurements have so far relied on the use of intense near-infrared laser pulses. In particular, attosecond streaking, laser-induced electron diffraction and high-harmonic generation all make use of non-perturbative light-matter interactions. Remarkably, the effect of the strong laser field on the studied sample has often been neglected in previous studies. Here we use high-harmonic spectroscopy to measure laser-induced modifications of the electronic structure of molecules. We study high-harmonic spectra of spatially oriented CH3F and CH3Br as generic examples of polar polyatomic molecules. We accurately measure intensity ratios of even and odd-harmonic orders, and of the emission from aligned and unaligned molecules. We show that these robust observables reveal a substantial modification of the molecular electronic structure by the external laser field. Our insights offer new challenges and opportunities for a range of emerging strong-field attosecond spectroscopies.

Figure 1. High-harmonic emission from aligned CH₃F and CH₃Br molecules.

Measured modulation of emission intensity in the ninth harmonic (H9) or H17 of an 800-nm driving field from CH₃F (a,b) and CH₃Br (c,d). (e,f) Calculated value of the alignment parameter 〈cos² θ〉 for CH₃F and CH₃Br using the experimental conditions (see Methods). The vertical lines in a–f mark the time corresponding to the classical period of rotation of the C–X axis. (g) Measured intensity ratio of emission from aligned (pump–probe delay of 20.1 ps) divided by isotropic molecules measured at different wavelengths and intensities. (h) Same as g for CH₃Br. The error bars correspond to twice the s.d. of the measured signal fluctuations.

Figure 2. High-harmonic spectra of oriented CH₃F molecules.

(a) High-harmonic spectrum of CH₃F molecules recorded at the instant of maximal orientation, that is, a delay of 19.5 ps between the two-colour pump and the 800-nm probe pulses (full red line) or for a nearly isotropic distribution (20.5 ps, dashed blue line). (b) Even-to-odd intensity ratio (2 × I_2n/(I_2n−1+I_2n+1)) at the instant of maximal orientation, that is, a delay of 19.5 ps. The error bars correspond to twice the s.d. of the measured signal fluctuations.

Figure 3. Effect of an electric field on the electronic structure of CH₃F.

The two components of the degenerate highest occupied molecular orbital (HOMO) of CH₃F from a HF//aug-cc-pVTZ calculation in the absence (a) or presence (b) of a static electric field of amplitude 0.05 a.u. (corresponding to a laser peak intensity of 0.88 × 10¹⁴ W cm⁻², isocontour value 0.1).

Figure 4. Laser-induced electronic-structure effects.

(a) Linear Stark effect of the HOMO of CH₃F for β=π/2. The displayed electronic eigenfunctions in a static electric field and correspond to linear combinations of the two field-free components φ^a and φ^b of the HOMO (see Equation (2)). The colours encode the relative sign of the wave functions. (b) Structure factors (see Equation (4)) determining the strong-field ionization rates obtained using the weak-field asymptotic theory for the strong-field ionization of two field-mixed components of the HOMO.

Figure 5. Comparison of high-harmonic intensity ratios with theoretical models.

(a) Aligned-to-anti-aligned ratios of CH₃Br compared with the two theories discussed in the text. The inset compares the experimentally observed alignment revival with the predictions of the two theories. (b) Same as (a) for CH₃F. (c) Even-to-odd ratios for CH₃F compared with the two theoretical models discussed in the text using 21.5% as degree of orientation. The error bars correspond to twice the s.d. of the measured signal fluctuations.