Photoelectron angular distributions from laser-excited aligned Yb atoms ionized by vacuum ultraviolet radiation

Using linearly polarized Ar I line radiation for ionization, the energyand angle-resolved photoemission from cw-laser-excited aligned Yb atoms in the (6s6p) 3P, state is studied. Photoelectron angular distributions have been measured for different orientations of the laser polarization vector relative to the polarization vector of the ionizing radiation. From polynomial fit coefficients for these distributions the ratio of reduced dipole matrix elements and the phaseshift difference for the transitions 6p+ E S and 6p+ Ed have been obtained at a photoelectron energy of 7.8 eV. In order to approach a complete description of the atomic photoionization process and to gain experimental access to the relevant dipole transition matrix elements and phaseshift differences it is necessary to acquire information exceeding total cross section data. For single-photon ionization of atoms in isotropic initial states the angular distribution of the photoelectron emission is described by the second Legendre polynomial and only a single asymmetry parameter p (Yang 1948). Further experimental information necessary for a more detailed description has to be gained from spin polarization measurements of the photoelectrons (Heinzmann 1980, Heckenkamp et a1 1986, Svensson et al 1988) or from photoion alignment as measured from the polarization or angular distribution of the fluorescence radiation (Kronast et a1 1986, JimCnez-Mier et a1 1986) or via Auger electron angular distribution (Southworth et a1 1983, Hausmann et a1 1988). For photoionization of polarized initial states, however, the description of the photoelectron angular distribution is more complex involving associated Legendre polynomials of higher order and containing several independent parameters which yield additional-and in some cases complete-dynamical information without requiring spin polarization analysis. A general theoretical expression for photoelectron angular distributions from polarized atoms was given by Klar and Kleinpoppen (1982). Such experiments can be performed in a two-step process in which a polarized state is prepared via resonant laser excitation and subsequently ionized. Up to now, photoelectron angular distributions from laser-excited states have been investigated only in several atomic systems where visible or ultraviolet laser radiation could still be used for ionization (Hansen et al 1980, Chien et a1 1983, Siege1 et a1 1983, Mullins et al 1985). In this letter, we present photoelectron distributions from a selectively laser-excited state using-in contrast-vacuum ultraviolet (vuv) radiation for photoionization. In 0953-4075/90/200629 +07%03.50 @ 1990 IOP Publishing Ltd L629 L630 Letter to the Editor this way, the photon energy range can be extended to regions well above the ionization threshold. In the last decade, the combination of cw laser excitation and ionization by vuv synchrotron radiation for the study of various metal atoms has provided a wealth of detailed spectroscopic information on excited atomic states with respect to resonant photoionization and partial cross section (Bizau et al 1985, 1986, Preses et al 1985, Nunnemann et al 1985, Cubaynes et al 1989). In particular, Meyer et a1 (1987) varied the alignment of the excited state (by changing the laser polarization direction or by selecting a different fine-structure component of this state) to study and classify core-excited autoionizing states. However, information on both dipole matrix elements and phases could not be extracted from these measurements, since for intensity reasons these experiments were all performed with angle-integrated collection of the photoelectrons. Here, we describe angle-resolved studies of non-resonant photoemission from optically aligned target atoms using linearly polarized vuv line radiation from an intense discharge lamp. Likewise, intense synchrotron radiation could be used for similar experiments; measurements at distinct photon energies for resonant ionization by Zimmermann and co-workers (Zimmermann 1990) are in progress. In our experiment, counterpropagating beams of linearly polarized laser and vuv radiation are used while fluorescence radiation and photoelectron emission is observed in a perpendicular direction. The set-up is represented schematically in figure 1. A boron nitride crucible is resistively heated to about 900 K to produce an effusive atomic beam of Yb with a density of about l o i 2 atoms/cm3 in the interaction region. The Yb atoms in the (6s)' 'So ground state are excited to (6s6p) 3P, by use of an actively stabilized single-mode ring dye laser at A = 555.8 nm (typical output power: 300 mW). The direction of the linear polarization of the laser (degree of polarization P 1.0) can be rotated with a Fresnel rhomb. Although Doppler broadening results in an effective linewidth of the transition of about 850 MHz, the isotope and hyperfine structure of the transition (Schuler and Korsching 1938) was resolved. This enabled selective excitation of the i ~ o t o p e s * ' ~ ~ Y b which are most abundant (-32%) in natural ytterbium. The resonance transition was saturated and we expect the fraction of excited atoms to be close to the limit of 16%. The fluorescence emitted from the excitation region centred -3.3 mm above the molybdenum nozzle of the furnace was imaged onto a photodiode using a lens and an aperture. For photoionization, a capillary discharge lamp (Schonhense and Heinzmann 1983) was operated with argon producing vuv line radiation at photon energies of 11.63 and 11.83 eV (Ar Ia and Ar rb, respectively). In order to record the photoelectron angular distribution, the vuv radiation is linearly polarized by a rotatable three-mirror reflecting SPECTROMETER ELECTRON t ROTATABLE REFLECTING POLARIZER APERTURE -L-LENS 6 PHOTOOIOOE Figure 1. Experimental set-up for measurements of photoelectron angular distributions and excited-state fluorescence. Letter to the Editor L63 1 polarizer (Hancock and Samson 1976) and the polarization vector is rotated relative to the detection direction of the electrons. With the method of Rabinovitch et a1 (1965), the degree of linear polarization was measured to be P = 0.91 at typical intensities of some 10" photons/s. Although the vuv lamp was operated with higher intensity at the helium resonance line the experiments reported here were carried out with argon rb due to the much higher 6p photoionization cross section at this photon energy. Photoelectrons emitted perpendicularly to the photon beams in an electricand magnetic-field-free region were energy-analysed with a simulated hemispherical spectrometer (Jost 1979) operated with a constant pass energy of 3.5 eV, at 100 meV resolution and with *5' angular acceptance and detected by a channeltron. Keeping the electron analyser fixed and rotating Evuv has the advantage that the source volume viewed by the spectrometer does not change provided the vuv polarizer is adjusted sufficiently well to avoid any displacement or deviation of the outgoing beam during rotation. The adjustment was done with the help of a helium-neon laser. In our experiment, the direction of the electric vectors EL and Evuv of both linearly polarised photon beams can be varied. The angle 0 between Evuv and the electron detection direction describes the photoelectron angular distribution. For completely linearly polarized light under our geometric conditions, the photoelectron angular distribution can be expressed in terms of associated Legendre polynomiais and five independent coefficients (Hansen et a1 1980): I ( @ ) = (Yo,+ (Y~OP2O(cos 0) f (Y4OP4O(cos 0) + (Y2IP2I(cOs 0) -k (Y41P41(cos 0). (1) Thus, four coefficients ark = can be determined from an angular distribution in relative measurements. These coefficients depend on the phase angle 77 between the two electric vectors EL and Evuv. Therefore the angular dependence is recorded preferentially by rotating both vectors simultaneously at constant phase angle. In general, the angular distribution depends not only on the transition matrix elements but also on the alignment of the laser-excited state as well as the degree of polarization of the light beams. atoms can in principle provide complete optical alignment, since only the (m, = 0) sublevel of the excited state is populated with linearly polarized light. However, at the relatively high target densities needed for observation of photoelectrons from the laser-excited state a depolarization due to radiation trapping and collisions of excited atoms occurs (Fischer and Hertel 1982). The effective alignment A was determined from the angular dependence of the resonance fluorescence at 555.8 nm measured with the photodiode while rotating the laser polarization direction (geometry c of Fischer and Hertel 1982)". Yb*(6s6p) 3P1 + hvvuv The transition 'So+ 3P1 for preparation of the excited In the experiments reported here we have examined the process: + [%+(6S) 2s1,2+e-(%/2, 4 / 2 9 4 , 2 ) 1 J = 0, 1,2. (2) The ionization potential for excited Yb3P, is 4.02eV (Martin et a1 1978). We have recorded photoelectron angular distributions at different phase angles with Ar Ib radiation for photoelectrons with a kinetic energy of 7.81 eV. The distribution for parallel electric vectors has already been reported (Kerling et a1 1989). Figure 2 shows t We use the definition A = a g h / F ( F + l), where aEh is the alignment parameter of Fischer and Hertel(l982) and F denotes the total angular momentum including the nuclear spin. For F = 1 , the alignment A is related to the state multipole moments pi according to: A = p 2 / ( f i po). L632 Letter to the Editor