Abstract
High harmonic generation in gases can be used as a probe of the electronic structure of the emitting medium, with attosecond temporal resolution and angström spatial resolution. The prospect of measuring molecular dynamics by pump–probe spectroscopy with such precision is attracting a lot of interest. An important issue in pump–probe spectroscopy lies in the ability to detect small signals: the detected signal can be easily dominated by the contributions from non-excited molecules or from a carrier gas. In this paper, we demonstrate that polarization-resolved pump–probe spectroscopy can be used to overcome this issue. We study high harmonic generation from rotationally excited molecules. We show that by measuring the harmonic field that is generated orthogonally to the driving laser field, the contrast in the detection of alignment revivals in nitrogen can be increased by a factor 4. We use this configuration to measure alignment revivals in an argon–nitrogen mixture, in which the total harmonic signal is dominated by the contributions from argon.
Highlights
In this paper, we study rotational excitation of molecular samples with high harmonic generation
Since the harmonic generation process is sensitive to the alignment of the molecule with respect to the laser polarization, revivals of molecular alignment can be probed by monitoring the high harmonic signal produced in the rotationally excited gas sample [11]–[13]
We have shown that polarization-resolved pump–probe spectroscopy could be used to study high harmonic generation from rotationally excited molecules
Summary
We study rotational excitation of molecular samples with high harmonic generation. Since the harmonic generation process is sensitive to the alignment of the molecule with respect to the laser polarization, revivals of molecular alignment can be probed by monitoring the high harmonic signal produced in the rotationally excited gas sample [11]–[13]. In order to analyze the polarization of the generated high-order harmonics, we placed a silver mirror with no protective coating at 45◦ incidence between the grating and the microchannel plates [23] This silver mirror acts as a polarizer: at this incidence, the ratio between the XUV reflectivity for S (φ = 0◦) and P (φ = 90◦) polarizations, i.e. its extinction ratio, is about 30. In the general case of an imperfect polarizer with extinction ratio A and XUV light polarized at an angle α from the probe laser pulse, the measured intensity follows:. The measured extinction ratio of our polarizer is about 30 at this wavelength
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