Energy Transfer into Molecular Vibrations and Rotations by Recoil in Inner-Shell Photoemission

A mixture of ${\mathrm{CF}}_4$ and CO gases is used to study photoelectron recoil effects extending into the tender x-ray region. In ${\mathrm{CF}}_4$, the vibrational envelope of the C $1s$ photoelectron spectrum becomes fully dominated by the recoil-induced excitations, revealing vibrational modes hidden from Franck-Condon excitations. In CO, using ${\mathrm{CF}}_4$ as an accurate energy calibrant, we determine the partitioning of the recoil-induced internal excitation energy between rotational and vibrational excitation. The observed rotational recoil energy is 2.88(28) times larger than the observed vibrational recoil energy, well in excess of the ratio of 2 predicted by the basic recoil model. The experiment is, however, in good agreement with the value of 2.68 if energy transfer via Coriolis coupling is included.

Figure 1

Carbon $1s$ photoelectron spectra of ${\mathrm{CF}}_4$, measured at different photon energies: (a) 330 eV, (b) 2.3 keV, (c) 3.0 keV, (d) 6.9 keV, and (e) 8.5 keV. Dots, experimental data points; continuous red line, least-squares curve-fitting result; vertical sticks, positions and relative intensities of the vibrational peaks.

Figure 2

Carbon $1s$ photoelectron spectra from a mixture of CO and ${\mathrm{CF}}_4$ gases, measured at (a) 2.3 and (b) 6.9 keV photon energy. The fitted positions and relative intensities of the vibrational peaks are marked by vertical lines, and the fitted total curves are shown by solid blue lines. Arrows indicate the distance between the $\nu =0$ peaks of ${\mathrm{CF}}_4$ and CO. Dashed lines connect the positions of the $\nu =0$ peaks in both spectra. The $x$-axis energy ranges are chosen to align the $\nu =0$ peaks of ${\mathrm{CF}}_4$.

Figure 3

Recoil energy partitioning into the total internal, rotational, and vibrational recoil excitations. Experiment: solid blue circles, total recoil energy; open red circles, rotational recoil; green diamonds, vibrational recoil. Recoil-model prediction including Coriolis coupling: solid lines; from highest to lowest, total, rotational, and vibrational recoil. Model prediction without Coriolis coupling: dashed lines.