Optics & Photonics News - February 2010

level lifetime—which, in the case of the H2 laser operating on transitions of the VUV Lyman bands of molecular hydrogen around 160 nm, implies rise-time requirements that are even faster than those of the N2 laser.

In 1926, James Franck of the University of Göttingen, Germany, suggested that, because the masses of the nuclei of a diatomic molecule are so much larger than the mass of an electron, the instantaneous positions of the vibrating nuclei cannot be significantly affected when an electron changes from one excited state to another by absorbing or emitting a photon. Thus, electronic transitions can be represented as vertical lines on a diagram of potential energy vs. internuclear separation. Within a few weeks, Franck’s principle was put on a sound quantum mechanical basis by Edward Condon of the University of California, Berkeley. He showed that, in addition, the most probable transitions occur at internuclear separations for which the overlap between the vibrational wavefunctions of the upper and lower level is maximized. This is known as the Franck-Condon principle.

When a fast moving electron impacts a molecule, the probability of excitation mirrors the optical transition probability. Thus, to a first approximation, electron impact excitation conforms to the Frank-Condon principle. As suggested by

Bazhulin and colleagues at the Lebedev Institute in Moscow,
a scheme based on the Frank-Condon loop can be used to cre-
ate transient population inversion in molecular gases. To illus-
trate this scheme in the case of H2, high-energy electrons in
the discharge preferentially excite H2 molecules in the ground
vibrational level (y0 = 0) of the ground electronic level X1S+g
to vibrational levels (y9 = 4–8) of the electronic level B1S+u.
The selective excitation produces short-lived inversions with
respect to excited vibrational levels (y0 = 11-14) of the ground
electronic level, which are themselves far too high in energy to
be populated at thermal energies. On much longer timescales,
the population in these states eventually returns to the ground
state by thermal collisions before the start of
the next discharge, thus completing a “loop”
in the energy diagram.
In 1970, R. T. Hodgson at the IBM Watson
Research Center first demonstrated VUV
laser action in H2. He observed oscillation
on several transitions in the Lyman bands
around 160 nm. This was followed in 1971
by Waynant’s observation of oscillation on two
transitions of the Werner bands at 116 and
123 nm using a traveling wave Blumlein
excitation circuitry developed by Waynant,
Shipman and colleagues at NRL Washington.

The molecular hydrogen lasers provided early evidence that gas lasers would be possible in short wavelength regions of the spectrum where most solid-state materials have limited optical transmission. Despite that, these systems never seem to have been

Nitrogen laser at the Clarendon Laboratory, Oxford.

Piper and Webb circa 1973

The breakdown voltage of molecular gases at the pressures needed for laser operation tends to be much higher than in low-pressure atomic gases. For this reason, the discharge in the N2 laser runs transversely across the width of the active volume rather than along its length. A capacitor bank charged to many tens of kilovolts and switched by a triggered spark gap (or, in more sophisticated devices, a thyratron) discharges its energy into the nitrogen at pressures from a few Torr to one atmosphere in a current pulse—which rises to several kiloamperes within 50 ns or less.

The output of the laser comprises a pulse that lasts some 10 ns for the lowest N2 pressures or less than 1 ns in the case of atmospheric pressure operation. Whatever the pressure, however, the vibrational states of the lower laser level have no way to get rid of the population in timescales of interest, and the laser pulse is therefore self-terminating; longer excitation pulses do not result in longer laser pulses.

Self-terminating VUV lasers—
The Franck-Condon Loop

This self-terminating feature is shared by many important gas laser systems, where the lower laser level is long-lived. It limits pulse length to about the upper-

As suggested by Bazhulin and colleagues at the Lebedev Institute in Moscow, a scheme based on the Frank-Condon loop can be used to create transient population inversion in molecular gases.