Stokes parameters and rovibrationally resolved cross sections for theX1Σg

Abstract

We report for the first time the calculation of Stokes parameters and state-to-state rovibrationally resolved differential cross sections for the excitation X 1 'E£(v = 0,N = 1) -> d 3 TL^(v = 0, N = 1) in H2 by electron impact at 25 eV. Comparison with the only available experimental data is encouraging.PACS numbers: 34.80.GsIn the last two decades there have been considerable experimental and theoretical efforts devoted to a better understanding of the dynamics of electron-impact excitation of atoms with the help of coherence and correlation parameters (CCP) [1][2][3][4]. The CCP can be measured in electron-photon coincidence experiments, where the emitted radiation from a specially selected ensemble of atoms or molecules is observed, namely, from those atoms or molecules that scattered the electrons into a well defined direction with a well defined energy loss [1,4]. Such an ensemble in general can be characterized by an alignment tensor ("an electric quadrupole moment" of its charge distribution) and by an orientation vector ("a magnetic dipole moment" describing its internal dynamical state) [1][2][3][4]. These features of the radiating ensemble of atoms or molecules manifest themselves in the intensity and polarization characteristics of the emitted radiation, quantitatively in the values of the measured Stokes parameters [1][2][3][4]. The comparison of CCP with theory is done at the level of complex excitation amplitudes and their interference, rather than at that of the usual excitation cross sections. Thus, while providing detailed information on the target state immediately after the collision, the CCP constitute a most sensitive test to theoretical models and approximations. Although the basic theoretical framework for the interpretation of an electronphoton coincidence experiment for molecules was laid down by Blum and Jakubowicz (BJ) in 1978 [5], its application is still very limited, since only a few such experiments have so far been reported [6-10]. McConkey et al.[9] demonstrated the feasibility of an electron-photon coincidence experiment involving rotationally resolved levels, and have reported Stokes parameters for the excitation of the d 3 U-(v = 0,iV = 1) level of H 2 . In addi-tion, rovibrationally resolved relative integral cross sections have been measured recently by Ottinger and Rox [11] for the excitation of the c 3 U~(v = 0,1, 2, 3, N = 2) levels of H2. On the theoretical side, however, no calculated CCP have ever been reported for molecules.In this Letter we report, for the first time, theoretical results for Stokes parameters and rovibrationally resolved differential cross sections (DCS) of a molecule. Specifically, we considered the electron-impact excitation of the d 3 n~(?;i = 0,N t = 1) level of H 2 from the X x Y^{vo = 0, iVo = 1) level at the incident energy EQ = 25 eV , where our theoretical results can be compared with the experimental data of McConkey et al [9].In what follows, we give a brief description of the formalism used, following BJ's notation: A will denote the com...

Anisotropy parameters for molecules excited by electron impact

“…McConkey et al [ 1 ] measured the Stokes parameters in electron-photon coincidence experiments for H2-transitions after excitation by electron impact. Meneses et al [2] obtained theoretical results for the same H2-transitions. Nordbeck et al [3] developed the general theory and calculated orientation and alignment parameters for various transitions in 0 2 using R-matrix elements obtained by Noble and Burke [4].…”