Exploring Non-covalent Interactions by Jet-Cooled Electronic …
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2.2.1 Thermal Heating
Thermal heating is the most convenient way to bring the sample molecules from the
solid/liquid state to the gas phase. Thus, the solid/liquid samples having moderate
vapor pressure at room temperature can be slightly heated to generate enough vapor
pressure for obtaining a good amount of signal in the supersonic jet experiment.
Generally, the sample is taken in a small stainless steel container placed behind the
pulse solenoid valve having an orifice of ~0.5–1 mm diameter [115]. The sample
holder, as well as the pulse valve, is heated at the required temperature using a resistive heater. The pulse valve is heated by 10–20 °C higher than the sample container to
avoid clogging the sample inside the pulse valve. However, the commercial solenoid
valves cannot be heated at a temperature higher than 110–120 °C. Moreover, nonvolatile samples, including biomolecules such as amino acids, peptides, etc. cannot
be heated at higher temperature as those will be fragmented upon heating. The alternative technique used for the vaporization of the non-volatile samples without any
fragmentation is laser desorption, which has been discussed in the next section.
2.2.2 Laser Desorption
Laser desorption circumvents the fragmentation of the fragile non-volatile sample
molecules as the heating is done using a narrow pulse width laser beam [116]. Thus,
the fragmentation of the samples is minimized by a significant reduction of the time
scale of the heating. Generally, a laser of nanosecond (ns) pulse width is used for the
desorption of the sample molecules. In a typical 10 ns laser pulse, the temperature
jump is about 1000 K with the rate of increment of the temperature ~10
11 K/s [68,
106, 117–119]. This technique is generally used to desorb the sample molecules from
the matrix of suitable material that assists the desorption [111, 117–126]. The matrix
should be a non-reactive, non-polar, and good heat-conducting material [106, 117,
124, 127]. In the case of the supersonic jet experiment, the sample is positioned after
the pulse valve. There are various ways to prepare the sample for laser desorption.
1. The sample could be dissolved in benzene or methanol, and the resulting mixture
could be coated on the surface of a metallic rod or pellet for the desorption [128].
However, a homogeneous film of the sample is preferable for the reduction of
the laser pulse to pulse fluctuation of the density of the sample vapor during the
laser desorption.
2. The fluctuation in the signal intensity due to significant variation in the sample
vapor in successive laser pulses can be reduced to some extent by dissolving the
sample in a liquid having high viscosity such as glycerol [124].
3. The solid sample could be ground to a fine powder and pressed homogeneously
on the surface of a graphite rod [111, 118, 119, 129–132].
4. The pulse to pulse fluctuation of the density of the sample vapor can be alleviated
to a significant extent by preparing a pellet by pressing a finely crushed mixture
of the sample and graphite powder (varied percentage) in a hydraulic press of
61
2.2.1 Thermal Heating
Thermal heating is the most convenient way to bring the sample molecules from the
solid/liquid state to the gas phase. Thus, the solid/liquid samples having moderate
vapor pressure at room temperature can be slightly heated to generate enough vapor
pressure for obtaining a good amount of signal in the supersonic jet experiment.
Generally, the sample is taken in a small stainless steel container placed behind the
pulse solenoid valve having an orifice of ~0.5–1 mm diameter [115]. The sample
holder, as well as the pulse valve, is heated at the required temperature using a resistive heater. The pulse valve is heated by 10–20 °C higher than the sample container to
avoid clogging the sample inside the pulse valve. However, the commercial solenoid
valves cannot be heated at a temperature higher than 110–120 °C. Moreover, nonvolatile samples, including biomolecules such as amino acids, peptides, etc. cannot
be heated at higher temperature as those will be fragmented upon heating. The alternative technique used for the vaporization of the non-volatile samples without any
fragmentation is laser desorption, which has been discussed in the next section.
2.2.2 Laser Desorption
Laser desorption circumvents the fragmentation of the fragile non-volatile sample
molecules as the heating is done using a narrow pulse width laser beam [116]. Thus,
the fragmentation of the samples is minimized by a significant reduction of the time
scale of the heating. Generally, a laser of nanosecond (ns) pulse width is used for the
desorption of the sample molecules. In a typical 10 ns laser pulse, the temperature
jump is about 1000 K with the rate of increment of the temperature ~10
11 K/s [68,
106, 117–119]. This technique is generally used to desorb the sample molecules from
the matrix of suitable material that assists the desorption [111, 117–126]. The matrix
should be a non-reactive, non-polar, and good heat-conducting material [106, 117,
124, 127]. In the case of the supersonic jet experiment, the sample is positioned after
the pulse valve. There are various ways to prepare the sample for laser desorption.
1. The sample could be dissolved in benzene or methanol, and the resulting mixture
could be coated on the surface of a metallic rod or pellet for the desorption [128].
However, a homogeneous film of the sample is preferable for the reduction of
the laser pulse to pulse fluctuation of the density of the sample vapor during the
laser desorption.
2. The fluctuation in the signal intensity due to significant variation in the sample
vapor in successive laser pulses can be reduced to some extent by dissolving the
sample in a liquid having high viscosity such as glycerol [124].
3. The solid sample could be ground to a fine powder and pressed homogeneously
on the surface of a graphite rod [111, 118, 119, 129–132].
4. The pulse to pulse fluctuation of the density of the sample vapor can be alleviated
to a significant extent by preparing a pellet by pressing a finely crushed mixture
of the sample and graphite powder (varied percentage) in a hydraulic press of