IR spectra of gaseous free radicals investigated in our laboratory using cavity ring down or time-resolved FTIR absorption are often severely overlapped with those of their precursors. Also, IR spectra of clusters of various sizes are similar. Without size selection or using double resonance, the overlapped spectral region is difficult to study. However, double resonance requires a chromophor that has an electronic state in the UV or visible region for resonant multi-photon ionization. It is thus strongly desirable to develop an IR technique that has mass selectivity so that the interference from other species may be eliminated. Thus, we set up a new system which can overcome this difficulty and has the following features.

  1. Having mass selectivity
  2. Having energy selectivity with selective ionization
  3. With high sensitivity

     In IR-VUV ionization, we employed VUV (118 nm or tunable) emission as a soft ionization light source; the photon energy was chosen to be near ionization threshold, so that molecules don’t further dissociate after ionization. Detection of ions with a time-of-flight system offers high sensitivity and mass selectivity.
Two excitation schemes for IR-VUV ionization are possible for investigation of neutral species.

Experimental setup

     VUV emission at 118 nm, from the 355-nm output of a Nd: YAG laser frequency-tripled in a mixture of Xe and Ar, served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission from an OPA/OPO system pumped with a second Nd: YAG laser served as a source of predissociation or excitation before ionization. Neutral clusters in the supersonic jet, softly ionized without fragmentation by laser light at 118 nm, are detected with a linear TOF mass spectrometer. By monitoring the variation in intensity of each cluster ion while scanning the wavenumber of the IR laser, introduced before the VUV laser pulse, we derived the raw spectra for each size-selected cluster. By processing of raw data based on photoionization efficiencies and the production and loss of each cluster due to predissociation, we can derive consistent IR spectra for the size-selected cluster.