AUTOIONIZATION RATES AND ENERGY LEVELS OF TRIPLET $v=1$ RYDBERG STATES OF PARA- AND ORTHO-$H_{2}$ MEASURED BY HIGH-RESOLUTION LASER $SPECTROSCOPY.^{*}$
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Date
1989
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Ohio State University
Abstract
We have measured the Lorentzian linewidths and absolute energy levels of 29 triplet Rydberg states of the $H_{2}$ molecule with quantum numbers $n=11$ to $22, v=1, R=0$ to $3, L=3$. This work provides the first measurements of autoionization rates of triplet nf Rydberg states. Measurements of the absolute energy levels of the Rydberg states relative to the metastable $c(2p)^{3}\Pi_{u}$ state are seven times more precise than the best previous work. An 18eV electron beam excites a collimated $H_{2}$ beam to the metastable $(2p)^{3}\Pi_{u}$ state. Stepwise excitation using two counterpropagating CW single-mode dye laser beams directed perpendicular to the molecular beam path excites the metastable molecule first to an intermediate state, $(3d^{3}\Sigma$ or $3d^{3}\Pi$), then to a triplet of state. While scanning the IR laser used for the last excitation, we detect $H_{2}^{+}$ ions to measure the lineshape and width. In addition we measure the energy to $0.0015 cm^{-1}$ of each laser beam (while they are set to the peaks of the transitions) using a travelling-arm interferometer (lambda-meter) referenced to a Zeemanstabilized He-Ne laser. Calculations based on the long range interaction of the Rydberg electron and the polarizability and quadrupole moment of the $H_{2}^{+}$ core predict the energy levels and autoionization $rates.^{1}$ For the $v=1$ Rydberg states we have measured, our data for the widths $(rate = 2\pi (width))$ and the energy levels agree well, except for a few well-understood exceptions. Using our measured energy levels we can also derive the ionization potential of the $c(2p)^{3}\Pi_{u}$ metastable state. We obtain the preliminary values: $\begin{array}{ll}IP(2p^{3}\Pi_{u}, v=1, N=1, J=2): & 27 137.108(7) cm^{-1}\\ IP(2p^{3}_{u}, v=1, N=2, J=3): & 27 022.078(6) cm^{-1}\end{array}$ The model predicts that rates for purely rotationally autoionizing Rydberg states ($nf, v=0, R=1$ to $4, L=3$) will be relatively larger; a study of these transitions is in progress.
Description
$^{\ast}$ Research supported in part by NSF grant PHY-8704527 + Bell Fellow 1 E. E. Eyler, Phys. rev. A, 34, 2881 (1986)
Author Institution: Lyman Laboratory of Physics, Harvard University
Author Institution: Lyman Laboratory of Physics, Harvard University