5 Time-Resolved Photoelectron Spectroscopy for Excited State Dynamics
107
nozzle, which is a sealed tube with a 50–500 µm hole. The source chamber housing the nozzle is separated from the laser-molecule interaction region by a molecular skimmer with a diameter of a few millimetres, generating a high density, wellcollimated molecular beam [60]. Such molecular beams have small transverse velocity and very low velocity spread in the propagation direction [61]. This reduces
the number of molecules in the interaction region which have been thermalized
through collision with the chamber walls and scattered back into the interaction
region. In addition, the expansion process cools down vibrational and rotational
motions in the molecules often leading to spectral simplification. The cooling effect
is a function of the nozzle diameter, the gas pressure behind the nozzle and the carrier gas [62, 63]. A liquid nitrogen cold trap is often placed after the laser-molecule
interaction region to absorb scattered molecules that pass through the interaction
region.
During a TRPES experiment the laser-molecule interaction occurs for only a tiny
fraction of the total time of the experiment, defined by the laser pulse duration and
laser repetition rate. Therefore, most of the gas from a continuous nozzle source
does not interact with the laser. The efficiency can be improved significantly by using pulsed nozzles. The pressure behind the pulsed nozzle can be as high as 100 bar
whilst maintaining reasonable vacuum conditions in the photoelectron spectrometer. Such high pressures can produce very efficient cooling; for example, Even-Lavie
pulsed nozzles can cool large molecules to temperatures below 1 K [64].
A more recent development is TRPES of liquid samples. Liquid jets were first
developed to study evaporation of molecular monomers and dimers [65]. For TRPES
in liquids, the key component is a quartz glass liquid nozzle with an aperture size
around 10 µm which generates a continuous flow of liquid; after travelling a few
millimetres in the vacuum the liquid jet breaks into droplets which are collected in a
trap [66]. Liquid jets have been employed successfully in UV-VUV/XUV [67] and
UV-UV [68] TRPES experiments.
5.3.3 Photoelectron Spectrometers
There are several different techniques for measuring photoelectron spectra. The
most popular photoelectron spectrometers are based on velocity map imaging (VMI)
or time of flight (ToF) methods.
ToF spectrometers measure the photoelectron spectrum by analysing the time
taken for an electron to travel from the interaction region to a detector. ToF spectrometers usually employ electrostatic or magnetic fields to guide the electrons to the
detector. A commonly employed ToF spectrometer is the magnetic bottle electron
spectrometer [69] which has the advantage of a large collection efficiency (typically
50 %). Photoelectrons created in the molecule-laser interaction region are guided
in a magnetic field toward the electron detector which is usually a microchannel
plate. ToF spectrometers can be constructed to have high energy resolution over a
wide range of photoelectron energies which is particularly useful for experiments
107
nozzle, which is a sealed tube with a 50–500 µm hole. The source chamber housing the nozzle is separated from the laser-molecule interaction region by a molecular skimmer with a diameter of a few millimetres, generating a high density, wellcollimated molecular beam [60]. Such molecular beams have small transverse velocity and very low velocity spread in the propagation direction [61]. This reduces
the number of molecules in the interaction region which have been thermalized
through collision with the chamber walls and scattered back into the interaction
region. In addition, the expansion process cools down vibrational and rotational
motions in the molecules often leading to spectral simplification. The cooling effect
is a function of the nozzle diameter, the gas pressure behind the nozzle and the carrier gas [62, 63]. A liquid nitrogen cold trap is often placed after the laser-molecule
interaction region to absorb scattered molecules that pass through the interaction
region.
During a TRPES experiment the laser-molecule interaction occurs for only a tiny
fraction of the total time of the experiment, defined by the laser pulse duration and
laser repetition rate. Therefore, most of the gas from a continuous nozzle source
does not interact with the laser. The efficiency can be improved significantly by using pulsed nozzles. The pressure behind the pulsed nozzle can be as high as 100 bar
whilst maintaining reasonable vacuum conditions in the photoelectron spectrometer. Such high pressures can produce very efficient cooling; for example, Even-Lavie
pulsed nozzles can cool large molecules to temperatures below 1 K [64].
A more recent development is TRPES of liquid samples. Liquid jets were first
developed to study evaporation of molecular monomers and dimers [65]. For TRPES
in liquids, the key component is a quartz glass liquid nozzle with an aperture size
around 10 µm which generates a continuous flow of liquid; after travelling a few
millimetres in the vacuum the liquid jet breaks into droplets which are collected in a
trap [66]. Liquid jets have been employed successfully in UV-VUV/XUV [67] and
UV-UV [68] TRPES experiments.
5.3.3 Photoelectron Spectrometers
There are several different techniques for measuring photoelectron spectra. The
most popular photoelectron spectrometers are based on velocity map imaging (VMI)
or time of flight (ToF) methods.
ToF spectrometers measure the photoelectron spectrum by analysing the time
taken for an electron to travel from the interaction region to a detector. ToF spectrometers usually employ electrostatic or magnetic fields to guide the electrons to the
detector. A commonly employed ToF spectrometer is the magnetic bottle electron
spectrometer [69] which has the advantage of a large collection efficiency (typically
50 %). Photoelectrons created in the molecule-laser interaction region are guided
in a magnetic field toward the electron detector which is usually a microchannel
plate. ToF spectrometers can be constructed to have high energy resolution over a
wide range of photoelectron energies which is particularly useful for experiments