Spectroscopic research in Amsterdam
The central research activity of the Amsterdam research group involves the spectroscopy and the photoionization dynamics of excited states of small stable and transient molecules, using pulsed lasers on ns, ps and fs timescales as excitation sources. Our spectroscopic method of choice is multiphoton ionization in conjunction with electron-kinetic-energy resolved photelectron spectroscopy. Much of our research in the field of "laser chemistry" is carried out from the viewpoint of preparing atomic and molecular ions in highly selected states (electronic, vibrational, rotational). Development of such methods is of central importance for various aspects of "state-selective chemistry".
Laser photoelectron spectroscopy possesses a number of unique features such as high sensitivity, high selectivity, the capability to observe orbital interactions in direct experimental fashion, and the possibility to monitor various exit channels in the decay of molecular excited states simultaneously. Molecules very suitable for study with our techniques are short-lived intermediates crucial in chemical processes in the earth's atmosphere under the influence of solar radiation, in combustion reactions, or which occur as interstellar species. In addition, larger molecules which can be viewed as building blocks for photonic materials can be profitably studied as well.
The group has developed and built two 'magnetic bottle' spectrometer systems. The first spectrometer, equipped with a simple effusive beam as sample inlet system, is used for resonance-enhanced multiphoton ionization (REMPI) studies with sensitive kinetic-energy-resolved electron detection (PES). The second 'magnetic bottle' spectrometer is dedicated to REMPI-PES studies in conjunction with a pulsed molecular beam sample inlet system.
The 'magnetic bottle' spectrometers are used in combination with either a nanosecond or a (sub)picosecond laser system and can operate in two complementary modes:
|
(I) |
As a function of the exciting laser wavelength all electrons generated in the photoionization process are counted regardless of their kinetic energies. In such a 'wavelength spectrum' in which the spectral resolution of ~ 0.1 cm -1 is determined by the laser, resonances occur which indicate the location of excited states. |
|
(ii) |
By tuning the laser wavelength to one of these resonances the ejected electrons can be analyzed according to their kinetic energies. In this way high-resolution (6-8 meV) photoelectron (PE) spectra of excited atomic and molecular states can be obtained for which the energetics of the photoionization process are known in detail |
The capabilities of our 'magnetic bottle' spectrometers also include mass-resolved ion detection, which has proved to be particularly useful in photodissociation studies, and pulsed-field ionization (PFI). With PFI and the subsequent detection of zero-kinetic-energy (ZEKE) electrons the limits of resolution in electron spectroscopy are pushed into the wavenumber regime.
Multiphoton ionization is well suited to obtain information on both the spectroscopy and the dynamics of excited molecular states. For such studies the availability of high-intensity pulsed laser excitation over a wide range of timescales is required. With the aim of spanning a timescale range of about six orders of magnitude, the research group therefore employs a nanosecond laser system consisting of an excimer laser which produces pump pulses of about 10 nanosecond duration, two dye lasers and frequency doubling accessories. Tunable light at wavelengths above 210 nm is available for multiphoton ionization studies. In addition the group also employs an advanced picosecond laser system. The picosecond laser system consists of a cw mode-locked Nd:YLF pump laser (Coherent Antares), a regenerative amplification stage (Continuum RGA-YLF 47-30) and two Pulsed Tunable Amplifiers (Continuum PTAs). Accessories for frequency tripling (to 351 nm) and for increasing the power of the Antares pump laser, as well as a new Satori (Coherent) dye laser with a pulse duration as short as 100 fs are available