Resonance-enhanced multiphoton ionization

The resonance enhanced Mehrphotonenionisation ( engl. resonance enhanced multi- photon ionization REMPI = ) is an ionization technique, are ionized by laser pulses in the molecules. The ions generated by this technique can be detected generally by methods of mass spectrometry, and thus used for chemical analysis.

Principle

Ionization of a molecule by REMPI is carried out in two steps, at each of which one or more photons are involved. The number of photons involved in the steps is indicated by the notation () REMPI. Do all photons involved the same wavelength, is spoken by a one-color REMPI at different photon energies of multi-color REMPI. The first step is the absorption of photons, and the resulting excitation of the molecule in a resonant state with a finite lifetime intermediate (often a Rydberg state ). The second step is to remove the ionization of the molecule by more photons from the intermediate resonant state. In this case, more energy is absorbed than is necessary for the achievement of ionization energy. In this case the excess energy is released as kinetic energy to the electrons leaving the molecule. The resulting ion can be detected by mass spectrometry, whereby the substance-specific absorption spectra of mixtures is possible.

The first step in reaching the excited bound states correspond to those, which are also studied by UV / VIS spectroscopy. A REMPI experiment thus provides data on both the UV / VIS spectrum, as well as on the mass spectrum of a substance and is therefore referred to as a 2- dimensional experiment.

Properties of the method

In REMPI is due to the result of the predetermined wavelength of the laser used photon energy, to produce a soft ionization method, ie, a large part of the molecule ions generated does not have enough surplus energy to fragment into smaller fragments.

The probability of Mehrphotonenionisation depends on the light intensity as follows from:

Stands for the nonlinearity of the process, which can be equated with the number of required photons. Thus, multiphoton processes require large light intensities. Illustrate this can be related to the fact that for an excitation with three photons they must arrive at the same molecule. Thus a REMPI process requires in comparison with the corresponding regular Mehrphotonenionisation less intense laser fields, since the required total number of photons absorbed in two steps. By using intense laser radiation a large amount of ions is therefore easily be generated.

Depending on the number of photons, the selection rules of the first step of which in UV / VIS - spectroscopy can distinguish. Thus, by REMPI, for example, transitions between states of the same parity (3s → 4s ) are observed when straight.

Selection rules

The band structure of a single electronic transition is seen in the picture on the right is an example. A REMPI spectrum consists of signals of many electronic transitions that can overlap energetically, so that the spectrum can be very complex. Signals belonging to the same electronic transition are classified depending on the change in the total angular momentum quantum number in branches. Here, transitions are associated with the letters ... O, P, Q, R, S ... following the alphabet. A person referred to in parentheses after the letter number represents the total angular momentum quantum number of the initial state, for example, S (2).

The absence of bands in a REMPI spectrum can have different causes. In the example transition missing, among other things, the R (0 ), Q ( 1 ) and Q ( 0) bands. The reason is readily apparent upon consideration of the term symbols of the states involved. The state has due to its electronic structure in the rotational ground state has a total angular momentum quantum number of. Thus, no transition is possible, which would have a total angular momentum quantum number of less than 2 in the target state.

The ionization step in the (2 1) REMPI a photon is absorbed and an electron ( Fermion, ) leaves the molecule. As selection rule for this step results in it. This explains that the generated ions occupy only a few rotational states.

Possible areas of application

The REMPI mass spectrometry assist in the examination of complex mixtures of several thousand chemical compounds and therefore is mainly used in the study of biological systems, such as energy sources, for example, fossil fuels or pyrolysis. The ionization can be found in the context of a very fast scanning mass analyzer varied use. Suitable mass analyzers are mainly various types of flight mass spectrometers, which differ mainly in the geometric structure. The selectivity for aromatic compounds makes the use of combustion processes such as waste incineration plants for the detection of toxic PAHs or dioxins possible.

The detection of trace components in flue gases of combustion processes takes place via a quartz tube, by which a portion of the gas is introduced as a mixture through a particulate filter and a heated deactivated fused silica capillary in the ion source. The UV laser is placed nearly orthogonal to the direction of entry. The ion beam is transferred via the ion optics in a time- of-flight mass spectrometer. By Substanzklasssenselektivität the first Anrgegungsprozesses so-called " REMPI " profiling is possible in the polyaromatic compounds such as anthracene or benzopyrene, easily can be ionized from the mixture out.

The method can also be used for the characterization of roasting gases in the coffee production. The roasted products are time-resolved recorded and statistically analyzed. With characteristic time - intensity profiles, a statement about the degree of roasting is made. Corresponding markers are mainly various derivatives of phenol and indole, and furfural. The Röstgradbeschreibung ranges from Cinnamoon to espresso degrees.

The possibility of REMPI ions in certain quantum states to generate a component of current research is to study with the aim of the kinetics of ion - molecule reactions, as a function of, for example, the rotational energy of the ion. The findings could help to make processes more efficient plasma chemistry.