Exploring Non-covalent Interactions by Jet-Cooled Electronic …
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solution phase UV-Vis/IR spectroscopy, the absorbance of the sample molecules is
directly monitored by measuring the intensity of the transmitted light with respect
to the incident light passing through the sample, as a function of the frequency of
the tunable UV-Vis/IR radiation [135]. The absorbance of the sample is given by the
following Beer-Lambert law:
A = εcl
where A is the absorbance of the sample, ε is the molar absorptivity or molar extinction coefficient of the sample, c is the concentration of the sample, l is the path length
traveled by the light in the sample. In the case of the supersonic molecular beam, it is
not possible to measure the insignificant change in the intensity of the incident light
after it passes through such an extremely diluted sample. Thus, it is required to have
alternative detection methods, which can be employed for the isolated gas-phase
spectroscopic techniques. It is the action spectroscopy, which can indirectly measure
the absorption of the UV and IR radiation by the sample molecules in the supersonic molecular beam [106]. Here, we will discuss various action spectroscopy techniques such as laser-induced fluorescence (LIF) excitation spectroscopy, resonantly
enhanced multiphoton ionization (REMPI) spectroscopy, UV-UV, and IR-UV holeburning spectroscopy, resonant ion-dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS) [111, 112]. Schematic diagrams of all
these different spectroscopic techniques have been shown in Fig. 2. Two major detection schemes are used to study gas-phase spectroscopy of isolated neutral molecules.
One of those is based on fluorescence, and the other one is on resonance-enhanced
multiphoton ionization.
2.3.1 Laser-Induced Fluorescence (LIF) Excitation Spectroscopy
LIF excitation spectroscopy is an indirect technique to measure the electronic absorption spectra of molecular systems studied in the supersonic jet [113, 136–139]. It
involves the excitation of the sample molecules from the lowest vibrational level (v"
= 0) of the ground electronic state (S 0 ) to different vibrational levels of the excited
electronic state (S 1 ) using a tunable UV laser. Total fluorescence from different vibrational levels of the excited electronic state is collected by a photomultiplier tube
(PMT) as a function of the excitation wavelength of the UV laser. Total fluorescence
measured from different vibrational levels of the S 1 state reveals the absorption of the
molecules from the v" = 0 level of the S 0 state to the corresponding vibronic levels.
LIF excitation spectroscopy provides vibrationally resolved electronic spectra of the
molecular systems, which yields the vibrational structure of the molecules in the S 1
state. LIF excitation spectroscopy is essential to measure the electronic absorption
spectra of the molecules which have very high ionization potential and hence are
difficult to study through resonance-enhanced multiphoton ionization spectroscopy.
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solution phase UV-Vis/IR spectroscopy, the absorbance of the sample molecules is
directly monitored by measuring the intensity of the transmitted light with respect
to the incident light passing through the sample, as a function of the frequency of
the tunable UV-Vis/IR radiation [135]. The absorbance of the sample is given by the
following Beer-Lambert law:
A = εcl
where A is the absorbance of the sample, ε is the molar absorptivity or molar extinction coefficient of the sample, c is the concentration of the sample, l is the path length
traveled by the light in the sample. In the case of the supersonic molecular beam, it is
not possible to measure the insignificant change in the intensity of the incident light
after it passes through such an extremely diluted sample. Thus, it is required to have
alternative detection methods, which can be employed for the isolated gas-phase
spectroscopic techniques. It is the action spectroscopy, which can indirectly measure
the absorption of the UV and IR radiation by the sample molecules in the supersonic molecular beam [106]. Here, we will discuss various action spectroscopy techniques such as laser-induced fluorescence (LIF) excitation spectroscopy, resonantly
enhanced multiphoton ionization (REMPI) spectroscopy, UV-UV, and IR-UV holeburning spectroscopy, resonant ion-dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS) [111, 112]. Schematic diagrams of all
these different spectroscopic techniques have been shown in Fig. 2. Two major detection schemes are used to study gas-phase spectroscopy of isolated neutral molecules.
One of those is based on fluorescence, and the other one is on resonance-enhanced
multiphoton ionization.
2.3.1 Laser-Induced Fluorescence (LIF) Excitation Spectroscopy
LIF excitation spectroscopy is an indirect technique to measure the electronic absorption spectra of molecular systems studied in the supersonic jet [113, 136–139]. It
involves the excitation of the sample molecules from the lowest vibrational level (v"
= 0) of the ground electronic state (S 0 ) to different vibrational levels of the excited
electronic state (S 1 ) using a tunable UV laser. Total fluorescence from different vibrational levels of the excited electronic state is collected by a photomultiplier tube
(PMT) as a function of the excitation wavelength of the UV laser. Total fluorescence
measured from different vibrational levels of the S 1 state reveals the absorption of the
molecules from the v" = 0 level of the S 0 state to the corresponding vibronic levels.
LIF excitation spectroscopy provides vibrationally resolved electronic spectra of the
molecular systems, which yields the vibrational structure of the molecules in the S 1
state. LIF excitation spectroscopy is essential to measure the electronic absorption
spectra of the molecules which have very high ionization potential and hence are
difficult to study through resonance-enhanced multiphoton ionization spectroscopy.