Ionization spectroscopy

Ionization spectroscopy monitors the absorption of photons on the molecular transition $E_i \rightarrow E_k$ by detecting the ions or electrons, produced by some means from the molecular excited state $E_k$. The necessary ionization of the excited molecule may be performed by photons, by collisions, or by an external electric, or magnetic field. On of the important process is photoionization of the $\vert k>$ state

$$M^*(E_k) + h\nu_2 \rightarrow M^+ + e^- .$$ (90)

The ionization photon may come either from the same laser which has excited the state $\vert k>$, or from a separate source, which can be another laser, of even an incoherent light source.

The main advantage of detecting electrons, or ions rather than photons is that the effectiveness of detection the charged particles is many orders of magnitude higher than than of photons. For instance, the current pulse produced by a single ion, or electron can be easily detected with a rather primitive galvanometer, while detection of a single photon by a photomultiplier is a complicated technical problem. More, it is technically easy to collect and detect all charged particles produced from ionization of the excited state $\vert k>$ using electrostatic optics, while effectiveness of detection of emitted photons is restricted by geometrical factors which usually do not allow to collect more that $10^{-3}$ from the total number of emitted photons.

Ionization methods find wide application in molecular spectroscopy and particularly, for determination of the internal-state distribution in reaction products of chemical reactions. These methods are in general called resonance-enhanced multiphoton ionization (REMPI) methods. When a one photon excitation is followed by a one-photon photoionization of the excited molecule, this method is called 1+1 REMPI. The method is very efficient for detecting molecules and molecular radicals, for instance, $OH$, $NO$, and others. However, many atomic products of chemical reactions, like $O$, $Cl$, $S$, $H$ and others have only high-lying excited states, corresponding to absorption of photons in the far UV spectral range where the conventional laser sources are not applicable. For detection of these species, a two-photon excitation is usually used on the first step with following one-photon photoionization of the excited atoms. This method is called 2+1 REMPI.

A very efficient photoionization process is the excitation of high-lying Rydberg energy levels of atoms, or molecules in the vicinity of the ionization limit which then decay by themselves (this process is called autoionization), or by applying an external electric field (this process is called field ionization):

$$M^*(E_k) + h\nu_2 \rightarrow M^{**} \rightarrow M^+ + e^- .$$ (91)

The absorption cross section for this process is generally much larger than that for the "bound-free" transition in eq. (90). Particularly, the process (91) is very efficient for detection hydrogen atom photofragments.

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