Experimental Observation of Rotational Doppler Broadening in a Molecular System

The first experimental evidence of rotational Doppler broadening in photoelectron spectra, reported here, show good agreement with recently described theoretical predictions. The dependence of the broadening on temperature and photoelectron kinetic energy is quantitatively predicted by the theory. The experiments verify that the rotational contributions to the linewidth are comparable to those from translational Doppler broadening and must be considered in the analysis of high-resolution photoelectron spectra. A classical model accounting for this newly observed effect is presented.

Figure 1

(a)  Variance of the distribution of rotational states populated in the ionization of ${\mathrm{N}}_2$ to form the $B$ state of ${\mathrm{N}}_2^{+}$ plotted against the kinetic energy of the photoelectron. The solid line is a straight line fitted to the data. Data from Poliakoff et al. [11]. (b)  Temperature derived from the ratio of the variance to the centroid.

Figure 2

Rotational contribution to the widths for the $B$-state peaks in the ${\mathrm{N}}_2$ photoelectron spectrum. The square of the rotational component of the width (full width at half maximum) is plotted against the photoelectron kinetic energy. The solid point is based on He I data from Ref. 19. The solid line is a straight line fitted to the data.