Exploring Non-covalent Interactions by Jet-Cooled Electronic …
65
is less than the ionization energy of the molecule. In that case, the second photon
of higher energy from a different laser fixed at a specific wavelength is used to
ionize the molecule while the wavelength of the first photon is tuned. However,
2nd photon energy is chosen so that the two-photon energy will be a few 100 cm
−1
higher than the ionization threshold of the molecule to prevent dissociation of the
molecule or the complex in the ionic state. This technique is called 2-color resonant
2-photon ionization (2C-R2PI) or (1 + 1
) REMPI spectroscopy [144, 145]. Here,
the two laser beams should be temporally as well as spatially overlapped, and the
pulse energy of the excitation laser beam should be as low as possible to avoid twophoton ionization from the first laser beam. The R2PI spectroscopy is coupled with
time of flight mass spectrometry to measure mass-selected electronic spectra of the
molecules and complexes.
2.3.3 UV-UV Hole-Burning Spectroscopy
This technique is used to discriminate the presence of different conformers of
molecules/complexes in the jet-cooled experiment [142, 146, 147]. Two UV lasers
are used in this experiment. Here, the first UV laser (pump/hole-burning laser of
pulse energy ~300–400 μJ) is scanned in the range of the electronic spectrum of the
molecule and the second UV laser (probe laser of pulse energy ~100–150 μJ), which
is fired 50–100 ns after the hole-burning laser, is kept fixed at a particular vibronic
transition of the electronic spectrum. The probe laser beam counter-propagates with
the pump laser beam and the two laser beams are spatially overlapped. The Pump
laser depletes the population of the ground state of the molecules, which is monitored by the probe laser. All the vibronic transitions belonging to the same conformer
show depletion in the intensity in the hole-burning spectrum as they arise from the
same ground state. In the REMPI spectrum, depletion is observed in the ion signal
of the probe laser, whereas depletion in the fluorescence signal of the probe laser is
observed in the LIF spectrum.
2.3.4 Resonant Ion-Dip Infrared Spectroscopy (RIDIRS)
This technique is used to measure conformation-specific infrared (IR) spectra of
molecules and complexes by probing their characteristic vibrational frequencies.
RIDIRS is an indirect IR absorption measurement technique, which is very important to determine the structures of different conformers present in the jet-cooled
experiment. In this experiment, counter-propagating UV and IR laser beams, which
are spatially overlapped, are mutually orthogonal to the molecular beam axis. The
pump IR laser is fired 100–200 ns prior to the probe UV laser, which is fixed to
a particular transition in the electronic spectrum (R2PI) [148–154]. The IR laser is
scanned in the region of the vibrational frequencies of the functional groups such as
–OH, –NH, –SH, –CH, C=O, etc. The UV ion signal depletes whenever the IR laser
frequency resonates with any vibrational frequency of the molecules or complexes.
65
is less than the ionization energy of the molecule. In that case, the second photon
of higher energy from a different laser fixed at a specific wavelength is used to
ionize the molecule while the wavelength of the first photon is tuned. However,
2nd photon energy is chosen so that the two-photon energy will be a few 100 cm
−1
higher than the ionization threshold of the molecule to prevent dissociation of the
molecule or the complex in the ionic state. This technique is called 2-color resonant
2-photon ionization (2C-R2PI) or (1 + 1
) REMPI spectroscopy [144, 145]. Here,
the two laser beams should be temporally as well as spatially overlapped, and the
pulse energy of the excitation laser beam should be as low as possible to avoid twophoton ionization from the first laser beam. The R2PI spectroscopy is coupled with
time of flight mass spectrometry to measure mass-selected electronic spectra of the
molecules and complexes.
2.3.3 UV-UV Hole-Burning Spectroscopy
This technique is used to discriminate the presence of different conformers of
molecules/complexes in the jet-cooled experiment [142, 146, 147]. Two UV lasers
are used in this experiment. Here, the first UV laser (pump/hole-burning laser of
pulse energy ~300–400 μJ) is scanned in the range of the electronic spectrum of the
molecule and the second UV laser (probe laser of pulse energy ~100–150 μJ), which
is fired 50–100 ns after the hole-burning laser, is kept fixed at a particular vibronic
transition of the electronic spectrum. The probe laser beam counter-propagates with
the pump laser beam and the two laser beams are spatially overlapped. The Pump
laser depletes the population of the ground state of the molecules, which is monitored by the probe laser. All the vibronic transitions belonging to the same conformer
show depletion in the intensity in the hole-burning spectrum as they arise from the
same ground state. In the REMPI spectrum, depletion is observed in the ion signal
of the probe laser, whereas depletion in the fluorescence signal of the probe laser is
observed in the LIF spectrum.
2.3.4 Resonant Ion-Dip Infrared Spectroscopy (RIDIRS)
This technique is used to measure conformation-specific infrared (IR) spectra of
molecules and complexes by probing their characteristic vibrational frequencies.
RIDIRS is an indirect IR absorption measurement technique, which is very important to determine the structures of different conformers present in the jet-cooled
experiment. In this experiment, counter-propagating UV and IR laser beams, which
are spatially overlapped, are mutually orthogonal to the molecular beam axis. The
pump IR laser is fired 100–200 ns prior to the probe UV laser, which is fixed to
a particular transition in the electronic spectrum (R2PI) [148–154]. The IR laser is
scanned in the region of the vibrational frequencies of the functional groups such as
–OH, –NH, –SH, –CH, C=O, etc. The UV ion signal depletes whenever the IR laser
frequency resonates with any vibrational frequency of the molecules or complexes.