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Spectroscopy Techniques: REMPI-TOF

Updated on July 27, 2017
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Saarith LM has a Ph.D in Physical Chemistry and is a teacher. He is also a father to two daughters that are energetic and fun.

What is REMPI-TOF?

Resonance Enhanced Multi-Photon Ionization - Time-of-Flight or REMPI-TOF for short is a highly sensitive spectroscopy technique which enables researchers to explore electron excitation mechanisms which are inaccessible via a more traditional single photon excitation.

REMPI-TOF is mainly utilized in the spectroscopy of atoms or small molecules. It was first used in 1978 by two research groups; Boels, et. al. in Germany and Antonov, et. al. from Russia. Its application has since spread in the scientific community, resulting in important breakthroughs in molecular spectroscopy.

A REMPI-TOF experiment can be considered a two-part process. First an atom or molecule needs to be ionized using REMPI techniques, which is then followed with a TOF measurement of the ionized particles. This allows for a highly detailed spectroscopy, measuring both frequency and mass spectra simultaneously.

From such detailed measurements, physical properties of the measured atoms or molecules such as spectroscopic constants, dissociation mechanisms and perturbation effects, can be discerned.

How Does the REMPI Part Work?

During a typical laser excitation of a particle, such as an atom or a small molecule, a single photon from the light source is absorbed by the particle. For the absorption to take place the energy of the absorbed photon must match an energy difference between two distinct energy levels of the particle.

The REMPI technique uses a high intensity laser to first excite and subsequently ionize atoms or molecules. Because of the high intensity of the light source, two or three photons can be simultaneously absorbed by an atom or molecule were the total energy of the absorbed photons matches an energy difference between two distinct energy levels of the particle.

The selection rules which govern electron excitation say that for each photon the change of total electron angular momentum during excitation must be either 0 or +/- 1. For multiphoton excitations, this rule is applied for each photon resulting in a wider range of measurable excited states otherwise impossible to detect.

For example, an excited electron state where the total angular momentum has increased by two from the ground state can be measured via a two-photon excitation.

Number of photons in excitation
Possible change in total electron angular momentum
1
0, +/-1
2
0, +/-1, +/- 2
3
0, +/-1, +/-2, +/-3

The excited particle can then be ionized by absorbing additional photons, the mechanisms of which can be either a straightforward single photon ionization or a more complex process involving several photons.

When discussing REMPI it is normal to signify how many photons are used to excite and then ionize the particle, e.g. (2+1) REMPI means that two photons were used to excite the atom or molecule followed with a single photon which ionizes the particle.

A REMPI spectrum of the HCl molecule. The spectrum shows several excitation peaks.
A REMPI spectrum of the HCl molecule. The spectrum shows several excitation peaks. | Source

How Does the TOF (Time-of-Flight) Part Work

If a group of different particles receive the same amount of kinetic energy the velocity of each particle will depend its atomic mass. Should these particles need to travel a fixed distance at constant velocity, the time each particle needs to cover the distance would therefore depend on their atomic mass, where the lighter particles would cover the distance in less time.

This simple fact is the core of Time-of-Flight mass spectroscopy. When atoms or molecules have been ionized they can be accelerated in an electric field, ensuring each ion gets the same kinetic energy. Should the ions leave the electric field they travel after that at a fixed velocity. By directing the particles to travel over a fixed distance towards a ion detector, the time from a laser pulse (ionization) till each ion hits the detector can be measured.

The time difference between the different ions over this fixed distance then create a mass spectrum, where the flight-time can be exchanged into atomic mass with a simple formula.

TOF = α√Mw + β

Where TOF is the flight-time of the ions, Mw is the atomic mass of the ion and α and β are constants dependent on the equipment used for time-of-flight measurement.

A simulated TOF array, displaying a repeller plate and extraction plates.
A simulated TOF array, displaying a repeller plate and extraction plates. | Source

This ability of REMPI-TOF to separate fragments which form during the excitation and ionization, means that laser frequency spectra of all the relevant masses can be measured simultaneously.

This also means that effects like perturbations are more easily spotted in both the laser frequency and mass spectra. Since the laser energy changes only slightly over the frequency difference between two peaks affected by perturbation, adjustments needed due to changing laser frequency is minimal. Therefore, perturbation comparisons are easier and more accurate.

Rotational spectra of different HCl fragments and isotopes. The unusual mass spectra profiles for both J'=2 peaks are an indication of perturbations.
Rotational spectra of different HCl fragments and isotopes. The unusual mass spectra profiles for both J'=2 peaks are an indication of perturbations. | Source

Equipment Used for REMPI-TOF Experiments

A typical REMPI-TOF setup consists of a high-powered, short-pulsed, excimer laser which acts as a power supplier to a tunable dye laser. The timescale of the excimer laser pulse is in the nanosecond scale and the power of each pulse is in the millijoule scale.

As the excimer laser beam enters the dye laser it excites a suitable dye depending on the frequency needed for the experiment. The dye laser then uses monochromatic gratings to tune the frequency of the laser.

The laser beam from the dye laser is then often directed through a second harmonic generator (SHG; frequency doubler) although this is not strictly necessary. The laser beam is then directed via prisms and focused into an excitation chamber which contains electric plating around the laser focal point, where the gas is ionized.

At pulsed intervals, which follows the laser pulse, a small amount of gas sample is injected into the chamber via a high velocity nozzle into the laser focal point. The gas crosses the laser beams focal point and part of it is ionized.

The electric plating then accelerates and directs the ionized particles to a Time-of-Flight tube at the end of which is a multi-channel plate detector which measures the ions and sends a signal to an oscilloscope that compiles the TOF mass spectrum for that particular excitation frequency.

Typical Setup of REMPI-TOF Equipment

A schematic of a typical setup for REMPI-TOF experiments.
A schematic of a typical setup for REMPI-TOF experiments. | Source

Summary

While REMPI-TOF is a powerful spectroscopy technique it is also a highly complex one and although I only touch on the main concepts of this technique I hope it gives the reader a clear picture of its application.

© 2017 Levictus Marcus Saarith

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