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
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
laser at the
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-
by Bazhulin and
colleagues at the
in Moscow, a
scheme based on
loop can be used
to create transient
in molecular gases.
22 | OPN Optics & Photonics News