Atoms and molecules have most of their oscillator strength in the vacuum ultraviolet (VUV)
and extreme ultraviolet (XUV), between the wavelengths of 200 nm and 30 nm. However,
most femtosecond spectroscopy has been restricted to the visible and infrared due to a
lack of sufficiently intense VUV and XUV femtosecond light sources. This thesis discusses
extensions of pump/probe spectroscopy to the VUV and XUV, and its application to the
dynamics of ethylene and oxygen molecules excited at 161 nm.
I begin with a detailed discussion of the short wavelength light source used in
this work. The source is based on the high order harmonics of a near infrared laser and
can deliver > 1010 photons per shot in femtosecond pulses, corresponding to nearly 10
MW peak power in the XUV. Measurements of the harmonic yields as a function of the
generation conditions reveal the roles of phase matching and ionization gating in the high
order harmonic generation process.
Pump/probe measurements are conducted using a unique VUV interferometer, capable
of combining two different harmonics at a focus with variable delay. Measurements of
VUV multiphoton ionization allows for characterization of the source and the interferometer.
In molecules, time resolved measurements of fragment ion yields reveal the femtosecond
dynamics of the system. The range of wavelengths available for pump and probe allows the
dynamics to be followed from photo-excitation all the way to dissociation without detection
The dynamics in ethylene upon π → π* excitation are protypical of larger molecules
and have thus served as an important test case for advanced ab initio molecular dynamics
theories. Femtosecond measurements to date, however, have been extremely lacking. In
the present work, through a series of pump probe experiments using VUV and XUV pulses,
time scales for the non-adiabatic relaxation of the electronic excitation, hydrogen migration
across the double bond, and H2 molecule elimination are measured and compared to theory.
In the simpler oxygen molecule, excitation in the Schuman-Runge continuum leads
to direct dissociation along the B 3Σu- potential energy curve. The time resolved photoion
spectra show that the total photoionization cross section of the molecule resembles two
oxygen atoms within 50 fs after excitation.