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J. Phys. Chem. 1996, 100, 8200-8203
Threshold for Photoionization of C6F6 in Solid Neon Bing-Ming Cheng Synchrotron Radiation Research Center, No. 1 R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu 30077, Taiwan, Republic of China ReceiVed: January 2, 1996; In Final Form: March 5, 1996X
The threshold for photoionization of C6F6 in solid neon was determined by irradiating a matrix sample in situ on scanning the energy of synchrotron radiation and by detecting the increased intensity of laser-induced fluorescence of the C6F6+ ion. A threshold energy of 10.08 ( 0.02 eV was measured for photoionization of C6F6 in solid neon at 4 K. Relative to the ionization energy in the gaseous phase, this value corresponds to a barrier of 0.17 eV due to the matrix cage. The barrier is a result not only of interaction of the ejected electron with a noble gas host but also of other factors.
Introduction Molecular ions are important because of their roles in the laboratory, in ionic reactions in the terrestrial atmosphere, and in interstellar phenomena. They also play significant roles in industrially important discharge systems such as for plasma etching and for chemical vapor deposition. Characterization of molecular ions is thus of considerable importance. However, molecular ions are transient species of a class that is particularly elusive and difficult to study.1 Matrix-isolation techniques have yielded valuable information about free radicals and other transient species.2-4 These techniques are applied extensively to acquire spectra of molecular ions to complement data of the gaseous phase.5-10 Of various methods to generate molecular ions in solid matrices, photoionization of suitable precursors deposited in a matrix by means of VUV radiation is the most direct approach. As noted in previous work, the solid matrix environment modifies the photophysical behavior of the guest molecules in many ways. The cage effect preventing molecular photodissociation is an important perturbation. An increased threshold energy of photodissociation of matrix-isolated molecules is typically observed. For example, for formation of OH on photodissociation of H2O in solid Ne, Ar, Kr, or Xe, energies 1.3-1.8 eV greater than that in the gaseous phase, 5.118 eV, were measured recently.11-14 As an altered photodissociation threshold is established for the condensed phase, the analogous question arises for photoionization. Red shifts are generally regarded as accompanying ionizing transitions in a solid medium.6 In a pure organic aromatic hydrocarbon crystal, the ionization energy of the crystal is typically less than that of the gaseous molecule.15 The molecular ion seems more stable in the crystalline environment because of the polarization of the surrounding molecules. This electronic polarization of the crystal by the cation results in less ionization energy required in most of the pure organic crystals. To verify whether this decreasing trend of ionization energy is still true in the solid matrix environment, we coupled a matrixisolation system to a synchrotron radiation source to assess the shift of a photoionization threshold in the solid matrix for the first time. A tunable and bright synchrotron radiation source instead of the invariant energy of VUV emission from hydrogen or other atomic resonance lamp in most previous work enables us to investigate an energy shift on photoionization in the condensed phase. In this experimental system a molecular ion X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00041-X CCC: $12.00
is detected by means of laser-induced fluorescence to enhance the sensitivity. The threshold for photoionization of CS2 in solid neon was measured at 10.31 ( 0.02 eV in the previous experiment;16 relative to the ionization energy in the gaseous phase, this value corresponds to a barrier of 0.23 eV. To elucidate this mysterious and unexpected result, we sought to study continuously the shift of ionization in a solid matrix. In the present work, we report the threshold for photoionization of C6F6 in solid neon. Of two reasons to select C6F6 for this work, the first is that C6F6 is a stable aromatic molecule, and another is that much spectral information about C6F6+ is available. An abundant knowledge of laser-induced fluorescence spectra of C6F6+ in the gaseous phase17-22 and matrices22-25 makes it well characterized and so suitable for our present project. Experimental Section We determined the threshold for photoionization by ionizing a matrix sample in situ on scanning the energy of synchrotron radiation and by detecting the increase in intensity of laserinduced fluorescence of the ion generated. The experimental arrangement is described in detail in ref 16. The ionizing light was conducted from the beam line (1-m Seya-Namioka monochromator) at the 1.3-GeV storage ring of the Synchrotron Radiation Research Center (SRRC) in Taiwan. To suppress the higher order light from the synchrotron radiation, a LiF filter window (2 mm thick) was used to separate the cryostat system from the beam line. Typically, during exposure of the solid sample the slit width of the beam line was set at 100 µm, corresponding to a nominal resolution of 0.25 nm. A mixed sample gas was deposited onto a rotatable LiF window maintained at 4 K in a refrigerator. The cold LiF window was set to face the gas inlet port. In this way the ionizing light from synchrotron radiation intersected this LiF window at an angle near 45°. To detect ions generated, the excitation beam of a laser (Nd:YAG/dye laser system) was incident in the direction opposite that of the light from the synchrotron. The two beams overlapped on the LiF window from opposite directions. Another monochromator (Jobin-Yvon, HR320) with a focal length of 0.32 m was put in the direction perpendicular to those of both the laser beam and the synchrotron radiation to disperse the emission of the ions. The fluorescent intensity of ions was detected with a photomultiplier equipped with a boxcar integrator system. To determine the threshold of photoionization, we fixed the laser at a particular excitation wavelength and set the dispersing © 1996 American Chemical Society
Threshold for Photoionization of C6F6 in Solid Neon
J. Phys. Chem., Vol. 100, No. 20, 1996 8201 TABLE 1: Comparison of Observed Line Wavenumbers (cm-1) and Assignments of Excitation of C6F6+ in Solid Neon with Results of Bondybey and Miller18
Figure 1. Excitation spectrum of C6F6+ in solid neon monitored at 0-0 emission.
monochromator at a particular emission feature of the ion. Then the fluorescent intensity of the ion was monitored by scanning the ionizing light from the synchrotron in the direction of decreasing wavelength (increasing energy) in steps of 0.25 nm. After exposing the solid sample to VUV radiation for 3-5 min at each step the fluorescent intensity of the cation C6F6+ was recorded. The molar ratio of precursor C6F6 and neon gas was typically 1/1000. Neon with a specified purity of 99.9995% was used directly without further purification, whereas C6F6 with a specified purity of 99.9% (Aldrich) was purified by vacuum distillation before use. The gas mixtures were prepared according to standard manomeric procedures in a UHV gashandling system. The rate of deposition was regulated between 90 and 120 µL/s and monitored with a flow transducer. The duration of deposition was generally 80-120 min. Results and Discussion The ionization threshold of C6F6 in the gaseous phase is 9.906 eV.26 To obtain LIF spectra of C6F6+ in solid neon, we arbitrarily set the wavelength of the synchrotron radiation at 118 nm (10.5 eV) to ionize the solid sample. Figure 1 shows an excitation spectrum of C6F6+ in solid neon after exposing the sample to VUV radiation for 1 h. The spectrum was recorded by incrementing the excitation wavelength of the dye laser at 0.02-nm (corresponding to 1 cm-1 at 460 nm) intervals in the wavelength range 432.0-464.5 nm. The dispersing monochromator was fixed at 476.0 nm (the vibrational line ν′′2 of the transition B ˜ 2A2u f X ˜ 2E1g) with a slit width of 40 µm. In front of the slit a filter (Schott GG-475) was placed to eliminate scattered light from the excitation laser. The gate window of the boxcar integrator was set at 0.1-0.25 µs. Each data point was integrated over 30 laser pulses by running at 20 Hz. The spectrum is uncorrected for the variable power of the dye laser. The observed features correspond to the vibrational structure ˜ 2E1g electronic transition of C6F6+ in solid of the B ˜ 2A2u r X neon. The excitation spectrum shows an origin at 21 559 cm-1 which is shifted 57 cm-1 to the red relative to its value for the gaseous phase (21 616.16 cm-1).21 This small shift (
