66
P. Panwaria and A. Das
The pulse energy of the IR laser beam used is ~2–3 mJ, while the UV laser pulse
energy is maintained at 200–300 μJ.
If the detection method used is fluorescence instead of multiphoton ionization,
then we measure the dip in the fluorescence signal instead of the ion signal, and the
technique is called Fluorescence Dip Infrared Spectroscopy (FDIRS) [154].
2.3.5 IR-UV Hole-Burning Spectroscopy
IR-UV hole-burning spectroscopy can also be used to discriminate the presence
of different conformers in the experiment. Thus, this technique provides the same
information, which is available from the UV-UV hole-burning spectroscopy. In this
technique, the IR laser, which is fixed at one of the vibrational transitions of the
molecule, precedes the UV laser by ~100 ns. As the IR laser depletes the population
of the ground state by the vibrational excitation of the particular mode, the intensity
of all the bands in the electronic spectrum belonging to the same conformer gets
reduced when the UV laser is scanned throughout the whole electronic spectral region
[106, 112]. On the other hand, the intensity of the electronic bands corresponding to
different conformers remains unaffected.
2.4 Experimental Set-Up for Jet-Cooled LIF
and Mass-Selected REMPI Spectroscopy
A schematic diagram of a generalized set-up for performing gas-phase laser spectroscopy experiments employing LIF and REMPI schemes coupled with supersonic
jet-cooling has been provided in Fig. 3 [115, 155]. The REMPI technique is further
combined with Time of Flight (TOF) mass spectrometry. The set-up comprises two
differentially pumped high vacuum chambers linked through a skimmer of 2 mm
diameter hole and 25 mm length. A commercial pulse solenoid valve (10 Hz rep.
rate, opening time ~200 μs) of orifice diameter of 0.5–1 mm attached with a stainless
steel sample holder connected with a long stainless steel pipe is mounted with an XYZ
manipulator. The whole pulse valve assembly is housed in the bigger chamber (supersonic expansion chamber) pumped by a 10-in. diffusion pump or similar capacity
turbomolecular pump. The XYZ manipulator is extremely important to align the
orifice of the pulse valve with respect to the skimmer, which is generally kept separated from the pulse valve by ~2.5 cm. A two-stage ion source consisting of a repeller
plate, an extraction grid, and an accelerating grid following Wiley-McLaren design
having a 1 m linear time of flight (TOF) tube is incorporated in the smaller size
vacuum chamber (ionization chamber) pumped by a 6-in. diffusion pump or a turbomolecular pump of similar capacity [156]. The TOF tube is placed perpendicular to
the axis of the molecular beam. A schematic of the jet-cooled LIF/REMPI Time of
Flight Mass spectrometry assembly is provided in Fig. 3.
P. Panwaria and A. Das
The pulse energy of the IR laser beam used is ~2–3 mJ, while the UV laser pulse
energy is maintained at 200–300 μJ.
If the detection method used is fluorescence instead of multiphoton ionization,
then we measure the dip in the fluorescence signal instead of the ion signal, and the
technique is called Fluorescence Dip Infrared Spectroscopy (FDIRS) [154].
2.3.5 IR-UV Hole-Burning Spectroscopy
IR-UV hole-burning spectroscopy can also be used to discriminate the presence
of different conformers in the experiment. Thus, this technique provides the same
information, which is available from the UV-UV hole-burning spectroscopy. In this
technique, the IR laser, which is fixed at one of the vibrational transitions of the
molecule, precedes the UV laser by ~100 ns. As the IR laser depletes the population
of the ground state by the vibrational excitation of the particular mode, the intensity
of all the bands in the electronic spectrum belonging to the same conformer gets
reduced when the UV laser is scanned throughout the whole electronic spectral region
[106, 112]. On the other hand, the intensity of the electronic bands corresponding to
different conformers remains unaffected.
2.4 Experimental Set-Up for Jet-Cooled LIF
and Mass-Selected REMPI Spectroscopy
A schematic diagram of a generalized set-up for performing gas-phase laser spectroscopy experiments employing LIF and REMPI schemes coupled with supersonic
jet-cooling has been provided in Fig. 3 [115, 155]. The REMPI technique is further
combined with Time of Flight (TOF) mass spectrometry. The set-up comprises two
differentially pumped high vacuum chambers linked through a skimmer of 2 mm
diameter hole and 25 mm length. A commercial pulse solenoid valve (10 Hz rep.
rate, opening time ~200 μs) of orifice diameter of 0.5–1 mm attached with a stainless
steel sample holder connected with a long stainless steel pipe is mounted with an XYZ
manipulator. The whole pulse valve assembly is housed in the bigger chamber (supersonic expansion chamber) pumped by a 10-in. diffusion pump or similar capacity
turbomolecular pump. The XYZ manipulator is extremely important to align the
orifice of the pulse valve with respect to the skimmer, which is generally kept separated from the pulse valve by ~2.5 cm. A two-stage ion source consisting of a repeller
plate, an extraction grid, and an accelerating grid following Wiley-McLaren design
having a 1 m linear time of flight (TOF) tube is incorporated in the smaller size
vacuum chamber (ionization chamber) pumped by a 6-in. diffusion pump or a turbomolecular pump of similar capacity [156]. The TOF tube is placed perpendicular to
the axis of the molecular beam. A schematic of the jet-cooled LIF/REMPI Time of
Flight Mass spectrometry assembly is provided in Fig. 3.