While
SHB
is a UV-UV double resonance technique, RIDIR works via monitoring a decrease of the
R2PI ion signal, if an IR- and the UV- transition share a common ground state level.
Compared to SHB there are no problems with intense ion signals from the IR burn laser. The technique can
be used to monitor the IR spectra of the water moiety in chromophore-water clusters, e.g. the important
OH stretch vibrations around 3 um.
Typically a grating-tuned LiNbO3
optical parametrical oscillator (OPO) pumped by the 1.06 um output of a Nd:YAG laser produces
1-2 mJ pulse-1 at about
1 cm-1 spectral resolution over the
2700-4000 cm-1 range necessary for excitation of different OH stretch vibrations.
Higher resolution (0.2 cm-1 ) can be obtained when the
OPO is seeded by pumping it with narrow bandwidth tunable IR radiation obtained from difference
frequency mixing of a dye laser and its 532 nm pump laser in a LiIO3 crystal.
The OPO and the IR beam path are flushed with nitrogen to avoid
ambient water absorption. IR and UV beam are aligned collinearly, or perpendicularly with
jet and IR-beam counterpropagating. The fluctuations in ion signal due to UV intensity changes
can be reduced by passing the UV laser through the ionization region twice with the IR laser
overlapping with only one pass. The two passes produce ions at slightly different ion source
potentials and therefore different arrival times at the detector. The two ion signals are divided
on a shot-to-shot basis.
The OH stretch vibrations in the S1 state can simply be obtained by pulsing the IR laser
simultanously or a few nanoseconds later than the
R2PI (or LIF) analysis laser.
