André J. A. van Roij

Gas-phase infrared photodissociation spectroscopy of cationic polyaromatic hydrocarbons

Infrared spectra of gas-phase cationic naphthalene, phenanthrene, anthracene, and pyrene are recorded in the 500-1600 cm-1 range using multiphoton dissociation in an ion trap. Gas-phase polyaromatic hydrocarbons are photoionized by an excimer laser and stored in a quadrupole ion trap. Subsequent interaction with the intense infrared radiation of a free electron laser that is tuned in resonance with an infrared-allowed transition of the ion leads to sequential multiphoton absorption facilitated by rapid intramolecular vibrational redistribution. Absorption of more than 50-100 infrared photons raises the internal energy to above the dissociation threshold, leading eventually to fragmentation of the ion. Mass selective detection of the cationic species stored in the trap yields the infrared absorption spectrum of the parent ion.

Cavity ring down spectroscopy on solid C60

The light absorption of a solid sample in the 8.5 μm region is measured via cavity ring down (CRD) absorption spectroscopy, using a free electron laser (FEL) as a source of widely tunable infrared (IR) radiation. A 3 mm thick zinc-selenide (ZnSe) window is used as a substrate for a 20–30 nm thick C60 film. On top of the structureless absorption due to ZnSe (<340 ppm), one of the well-known fundamental IR absorption lines of C60 is measured with monolayer sensitivity.

Photodissociation of the OD radical at 226 and 243 nm

The photodissociation dynamics of state selected OD radicals has been examined at 243 and 226 nm using velocity map imaging to probe the angle–speed distributions of the D(2S) and O(3P2) products. Both experiment and complementary first principle calculations demonstrate that photodissociation occurs by promotion of OD from high vibrational levels of the ground X 2Π state to the repulsive 1 2Σ− state.

Infrared resonance enhanced multiphoton ionization of fullerenes

Gas-phase fullerenes are resonantly heated by a train of high power subpicosecond pulses from a Free Electron Laser (FEL) to internal energies at which they efficiently undergo delayed ionization with little or no fragmentation [1]. When the laser is tuned from 6–20 μm while the amount of laser produced parent ions is recorded, resonant absorption of 200–600 IR photons is observed and the resulting wavelength spectra have a close resemblance to the IR absorption spectrum of C60. InfraRed Resonance Enhanced Multi Photon Ionization (IR-REMPI) with a FEL enables extremely sensitive IR spectroscopy with mass selective detection of gas-phase fullerenes. To gain insight into the ionization mechanism, the time evolution of the production of the ions following IR excitation is recorded. Ion creation is unexpectedly slow, occurring at times beyond 0.1 ms after laser excitation. The results indicate that reaching the first electronically excited state is a high hurdle on the way from hot molecules to ions.