Spectrocopy of an Unstable Molecule


     The matrix-isolation technique has served to produce and to trap novel chemical species in diverse chemical and physical problems since its inception in 1954.1 Several books on matrix-isolation techniques and their applications have covered various aspects of this simple yet useful technique. In addition to its capability of trapping and preserving unstable species, the matrix-isolation technique has an additional advantage in accumulating the sample via continuous deposition for a protracted period to facilitate detection of weak absorption lines due to a small cross section or small concentration. Furthermore, the sample required in the experiment is small relative to that in gas-phase experiments; use of precious samples such as isotopic variants and specially synthesized precursors thus becomes feasible.

     The photochemical behavior of species isolated in matrices might be distinct from that in the gaseous phase. In the gaseous phase at small pressure, fragments have little chance to collide with each other upon photodissociation, whereas photofragments produced in a matrix are typically constrained within the matrix cage; consequently they might collide with each other several times at various impact angles and eventually react with each other. For these types of “cage” reactions, the chance for photofragments to collide with each other within a certain acceptance cone is much greater than in the gaseous phase; hence the possibility of producing various isomers of the original precursor or other stable products or their complex is much increased. The low temperature and the efficient relaxation of energy by the matrix host further enhance the formation of unstable intermediates that are producedwith difficulty in the gaseous phase.


     Several cryogenic systems were used. The closed-cycle helium refrigeration systems (Air Products), capable of cooling to 12 K and 5 K, were employed for use with Ar, Kr, Xe or N2 matrices, and with a Ne matrix, respectively. We recently replaced those systems with cryogenic systems (Sumitomo) capable of cooling to 3.6 K, The substrate for the matrix sample is a copper block plated with nickel or platinum to reflect the incident (IR or visible) beam to a detector. Typically 0.01 mol of a sample mixture is deposited over a period of 2-6 hours;the concentration of guest molecules is typically 1/3000-1/100.
One system is designed so that IR and UV/visible absorption spectra of the same matrix sample can be recorded consecutively with a Fourier-transform spectrometer (Bomem DA8). IR absorption spectra covering the spectral range 500-4000 cm-1 are recorded with a globar source, a KBr beamsplitter, and a HgCdTe detector cooled to 77 K, whereas UVvisible absorption spectra covering part of the spectral range of 300-800 nm are recorded with a deuterium lamp, a quartz beamsplitter, and a photomultiplier. Typically 300-600 scans are collected for IR measurements at a resolution 0.5 cm-1, and 2000 scans are collected for UV-visible measurements at a resolution 2.0 cm-1. The spectrometer was evacuated except for the optical path between the instrument, the cryogenic system, and the detector, which was purged with dry nitrogen.
Several lasers are employed for photolysis. Typically a fixed wavelength laser, such as excimer lasers (ArF at 193 nm, KrF at 248 nm, and XeCl at 308 nm) and a Nd: YAG laser (1064, 532, 355, and 266 nm) are used. A tunable laser, such as a dye laser pumped with a XeCl or a Nd: YAG laser or an OPO (optical parametric oscillator) pumped with a Nd: YAG laser, is employed for fluorescence excitation or, photolysis.

     Sulfur dioxide (OSO) isolated in solid argon at 13 K was irradiated with light at 193 nm from an ArF excimer laser. Weak absorption lines at 1006.1, 1004.7, and 739.9 cm-1 observed afterphotolysis are assigned to sulfur superoxide ~SOO.
     Linear carbon disulfide (denoted as SCS) isolated in solid N2 or Ar at 13 K was irradiated with light at 193 nm from an ArF excimer laser. In addition to an absorption line of CS at 1277.4 cm-1, new lines at 881.3 and 520.9 cm-1 were observed after photolysis of SCS in solid N2. These lines are assigned to cyclic CS2 (denoted cyc-CS2).

     Irradiation of a Ne matrix sample containing NO and CO near 4 K with an ArF excimer laser at 193 nm yielded new lines at 2045.1 and 968.0 cm−1 that were depleted upon secondary photolysis at 308 nm. These lines are assigned to C=O stretching and mixed stretching modes of ONCO.


  1. Isomers of SO2: infrared absorption of SOO in solid argon, L.-S. Chen, C.-I Lee, and Y.- P. Lee, J. Chem. Phys. 105, 9454 (1996).
  2. Production and IR Absorption of Cyclic CS2 in Solid Ar, M. Bahou, Y.-C. Lee, and Y.-P. Lee, J. Am. Chem. Soc. 122, 661 (2000).
  3. Isomers of S2O: IR absorption spectra of cyclic S2O in solid Ar, W.-J. Lo, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 117, 6655 (2002).
  4. Ultraviolet absorption spectrum of cyclic S2O in solid Ar, W.-J. Lo, Y.-J. Wu, and Y.-P. Lee, J. Phys. Chem. A. 107, 6944 (2003).
  5. Isomers of HSCO: IR absorption spectra of t-HSCO in solid Ar, W.-J. Lo, H.-F. Chen, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 120, 5717 (2004).
  6. Isomers of OCS2: IR absorption spectra of OSCS and O(CS2) in solid Ar, W.-J. Lo, H.-F.Chen, P.-H. Chou, and Y.-P. Lee, J. Chem. Phys. 121, 12371 (2004).
  7. Isomers of NCO2: IR absorption spectra of ONCO in solid Ne, Y.-J. Wu and Y.-P. Lee,J. Chem. Phys. 123, 174301 (2005).