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Higher Excited States in Multiphoton Photochemical Reactions “Hint” Toward Rapid Chemistry
F
In Figure 1, the spectrum is structured until about 9 eV where ionization occurs. Thereafter, a continuum is observed due to the ionization process. In solutions, ionization is noted well below the gas-phase ionization potential of 9 eV. In a thermodynamic sense, the lowering of the ionization energy in the bulk phase is due to the polarization energy of the medium and the lower-energy state of benzene in the bulk where the electron resides. This energy difference can be as large as 2 eV, thus bringing the onset of photoionization down into the nearUV. The mechanism now gets complex but clearly shows the rich chemistry and the new mechanisms of excitation into higher-energy states. Similar processes are induced by twophoton laser excitation, where only the state equivalent to the total energy of the process is excited. Many liquids show this effect, but for brevity, only carbon tetrachloride and water will be discussed. Multiphoton events have been observed by Mataga et al.3 in many liquids such as cyclohexane, benzene, and CCl4. The data are reminiscent of the radiolysis of these liquids, where solvated electrons are produced in bulk C6H12 and a CCl4+ cation is produced in bulk CCl4. The prominent feature is ionization of the liquid, both in radiolysis and two-photon excitation. A detailed study4 shows the same general principles by both methods of excitation, but the kinetics of the observed ions differ in the two methods of excitation. For example, twophoton excitation of CCl4 produces the solvent-separated ion pair, CCl3+∥Cl−. This species is formed following reactions 1 and 2.4
rom early in the 20th century, it was realized that light played a crucial role in many chemical events, for example, photosynthesis, vision, photography, and so forth. The procedure until the 1950s was to shine light on a system and measure products. The light source of choice, for much research, was the mercury lamp, which under suitable conditions produces light of wavelength 184, 254, 365, and so forth. Many texts cover this interesting era of steady-state photochemistry. Just after World War II, Sir George Porter (1967 Nobel Prize in Chemistry) made a significant advance in the field by the successful use of pulsed light sources, which observed events on the 10−3 s time scale, that is, formation of intermediary species in photolysis. This technique of flash photolysis, rather than the earlier steady-state photolysis, was quickly given a great boost by the development of lasers, which improved the response time of observation to 10−9 s and, shortly thereafter, to 10−15 s. Lasers also enabled tuning of several new wavelengths for excitation. Indeed, the kinetic part of photochemistry is now very well developed. However, little has been done on creating higher excited states and following their subsequent chemical processes. An early example clearly illustrates the different products produced upon excitation of various states of benzene.1 Several products are produced using different exciting wavelengths, with some complication due to secondary excitation of the products, such as fulvene, benzvalene, and cis-1,3-hexadien-5yne. Initial photolysis of benzene at 185 nm produces large yields of fulvene, along with cis-1,3-hexadien-5-yne. This is one of the simplest studies using varying wavelengths and energy states in benzene, a molecule of both industrial and academic interest. Because of the short wavelengths involved, these studies are difficult, requiring spectroscopy cells of high purity special quartz and so forth. The additional care needed for experimentation may have prevented many from carrying out such studies. In the early days, it was always assumed that most molecules, when excited into higher excited states, rapidly (12 eV required in the gas phase. Multiphoton processes were also observed at photon energies of 3 and 4 eV.6 Excitation at the low-energy +
Published: August 7, 2014 2586
dx.doi.org/10.1021/jz501173p | J. Phys. Chem. Lett. 2014, 5, 2586−2587
The Journal of Physical Chemistry Letters
Guest Commentary
Figure 1. Electron impact spectrum of benzene. Accelerating voltage = 50. To excitation energies of approximately 3 eV, nothing is found that significantly exceeds the background. (Reproduced with permission from ref 2. Copyright 1968, AIP Publishing LLC.) Phenomena IV; Auston, D., Eisenthal, K., Eds.; Springer: Berlin, Heidelberg, Germany, 1984; Vol. 38, pp 317−319. (4) Zhang, G.; Thomas, J. K. The Laser Two-Photon Photolysis of Liquid Carbon Tetrachloride. Photochem. Photobiol. 2006, 82, 158− 162. (5) Sander, M. U.; Luther, K.; Troe, J. Excitation Energy Dependence of the Photoionization Energy of Liquid Water. J. Phys. Chem. 1993, 97, 11489−11492. (6) Bartels, D. M.; Crowell, R. A. Multiphoton Ionization of Liquid Water with 3.0−5.0 eV Photons. J. Phys. Chem. 1996, 100, 17940− 17949.
end produced no geminate recombination of the hydrated electron, while at higher energies up to 15%, it showed an initial fast decay in the picosecond region. At first sight, the photoinduced processes appear to be very complicated, but the above observation simply implies lowering of the ionization energy, Ig, by the bulk water, an important issue in itself. The multiphoton excitation of liquid water is a convenient technique to produce ionized products. Two-photon excitation and radiolysis produce higher excited states of liquids. However, these states are extremely reactive and eventually yield longer-lived products. This speaks strongly against both methods to produce higher excited states but nevertheless clearly shows the rich chemistry of these more excited states. Methods of exciting into higher excited states seem to be more of a challenge than initially envisaged. The usual experimental difficulties are present, for example, transparent irradiation cell windows and so forth. However, really high energies are only achieved by energetic electrons.1 Hence, it is found that the mechanism of energy loss is quite different from that used in photochemistry, and the realm of radiation chemistry begins. Even if, as in multiphoton photolysis, the higher excited state can be achieved “cleanly”, then vast differences emerge between the gas phase and the condensed liquid phase. Perhaps the answer lies in studying reactions of higher excited states following the first production of the energetic state and placing the emphasis there.2
J. Kerry Thomas*
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Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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REFERENCES
(1) Kaplan, L.; Wilzbach, K. E. Photolysis of Benzene Vapor at 1849 A. Formation of cis-1,3-Hexadien-5-yne. J. Am. Chem. Soc. 1968, 90, 5646−5647. (2) Lassettre, E. N.; Skerbele, A.; Dillon, M. A.; Ross, K. J. High Resolution Study of Electron Impact Spectra at Kinetic Energies between 33 and 100 eV and Scattering Angles to 16°. J. Chem. Phys. 1968, 48, 5066−5096. (3) Miyasaka, H.; Masuhara, H.; Mataga, N. Picosecond Multiphoton Laser Photolysis and Spectroscopy of Liquid Benzenes. In Ultrafast 2587
dx.doi.org/10.1021/jz501173p | J. Phys. Chem. Lett. 2014, 5, 2586−2587
