Experimental Observation of Rotational Doppler Broadening in a Molecular System T. D. Thomas, 1 E. Kukk, 2 K. Ueda, 3 T. Ouchi, 3 K. Sakai, 3 T. X. Carroll, 4 C. Nicolas, 5 O. Travnikova, 5 and C. Miron 5 1 Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, USA 2 Department of Physics, University of Turku, FI-20014 Turku, Finland 3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan 4 Division of Natural Sciences, Mathematics, and Physical Education, Keuka College, Keuka Park, New York 14478 5 Synchrotron SOLEIL, l’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France (Received 16 December 2010; published 13 May 2011) 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. DOI: 10.1103/PhysRevLett.106.193009 PACS numbers: 33.60.+q, 33.70.Jg The Doppler effect is well known in physics. It is seen principally as the consequence of the linear motion of an object and is observed in many phenomena such as the redshift of radiation from receding stars, the measurement of velocities via radar, the broadening of lines in atomic and molecular spectroscopy, and the shifting of nuclear transi- tion energies in Mo ¨ssbauer spectroscopy. Less well known is the rotational Doppler effect, where the rotational motion of an object affects the energy of emitted radiation. This is seen primarily in astrophysics, where it is used to determine angular velocities of astronomical objects. To our knowl- edge, it has never been observed at the molecular level [1–3]. In a recent theoretical investigation of this phenome- non, Sun, Wang and Gel’mukhanov [4] have predicted that rotational Doppler broadening should be observable in high-resolution photoelectron spectroscopy and have pre- sented expressions to describe its dependence on the kinetic energy of the photoelectron and on the temperature of the sample. Here we present the first experimental investigation of this recently predicted phenomenon. In the 40þ years since Siegbahn and coworkers demon- strated the unique ability of electron spectroscopy to probe electronic structure and chemical environment in mole- cules and solids [5], impressive progress has been achieved in this field [6,7], and, in particular, in the instrumental resolution. However, there remains line broadening due to intrinsic sources, such as the Doppler effect, and, in the case of inner-shell electrons, the natural lifetime. With the possibility that the Auger-Resonant-Raman effect can be used to overcome lifetime broadening in some situations [8], it is the Doppler broadening that sets the ultimate limit to the narrowest obtainable linewidth in photoelectron spectroscopy [9]. For instance in a recent measurement of the 4p photoelectron spectrum of Kr [10] the observed linewidth of 66 meV arises from an instrumental resolution of 59 meV and a translational Doppler broadening of 30 meV. For molecules, there can be additional broaden- ing. In the same experiment, the peaks arising from valence ionization of N 2 have widths of about 90 meV, which is more than can be accounted for by the instrumental reso- lution and the translational Doppler broadening (52 meV). There is an almost equal additional broadening of 45 meV, which can be attributed to the effect of rotational motion of the molecule. This rotational broadening is our concern here and we present the results of measurements covering a wide range of photoelectron kinetic energies on room- temperature samples. Combining these results with others from low-temperature experiments [11] allows us to con- firm the dependence on both kinetic energy and tempera- ture predicted by Sun et al. [4]. We also show that a simple classical model gives results that are in agreement with both the quantum-mechanical model of Sun et al. and with our experimental results. These results are significant in a number of ways. First is the demonstration for the first time that Doppler broad- ening due to rotational motion can be seen in a molecule and that this effect can be observed via high-resolution photoelectron spectroscopy. Second, we find experimen- tally that if we have accounted for instrumental broad- ening, translational Doppler broadening, and rotational Doppler broadening, then we have accounted for all of the major sources of broadening. Third, as noted by Sun et al., this broadening will be a common feature of such spectra, occurring whenever the site of photoemission differs from the center of mass of the molecule. Fourth, as the example of N 2 cited above and the examples given by Sun et al. indicate, the rotational Doppler broadening can be comparable to, and, in some cases, even greater than the translational Doppler broadening [12,13]. Thus any analysis of the line shape in such spectroscopy must take this effect into account. Fifth, we show that a simple classical model can account quantitatively for this effect, PRL 106, 193009 (2011) PHYSICAL REVIEW LETTERS week ending 13 MAY 2011 0031-9007= 11=106(19)=193009(4) 193009-1 Ó 2011 American Physical Society