Photochemistry of Small Molecules

The classic text book in photochemistry is that of Noyes and Leighton, The Photochemistry of Gases published in 1941. In 1966, a comprehensive volume by Calvert and Pitts brought the field up to date, but it soon became obvious that the field of photochemistry was expanding too rapidly to be covered in a single comprehensive volume. In fact, in the latter part of the 20th century organic photochemistry and physical photochemistry existed as separate disciplines with largely separate constituencies. Organic photochemistry included such molecules as methane (CH4), whereas NO and other oxides of nitrogen are inorganic. The book by Hideo Okabe, Photochemistry of Small Molecules [1], published in 1978, is so titled to convey the author’s intent to include both organic and inorganic molecules, but to direct his attention to simple molecules of both genres. The author’s definition of “small” as five or fewer atoms is arbitrary and, unfortunately, excludes much of his own seminal work on the photochemistry of ethane and propane, which contain eight and eleven atoms respectively. In his Photochemistry of Small Molecules Okabe provides an elegant discussion of the spectroscopy of atoms and molecules as well as of the rotational, vibrational, and electronic energy states. In photochemical research, small molecules often have discrete electronic spectra, and any discussion of mechanisms of photochemical reactions must be consistent with these spectra. Accordingly, Okabe gives special attention to the details of what was known about the spectra. He systematically presents chapters on photochemistry and spectroscopy of diatomic, triatomic, and polyatomic molecules (i.e., four and five atom) in Chapters V, VI and VII of the book. His discussion of the basis of photochemical correlation rules and selection rules is very thoroughly presented. His knowledge of the relevant spectroscopy is at a very high level, and his incorporation of spectroscopic aspects into his discussion of the photochemistry, if not unique, is certainly extraordinary. Okabe’s subject, the photochemistry of small molecules, came into prominence in the 1960-1980 period just at the time when planetary modelers were struggling to provide a quantitative understanding of the chemistry of the atmospheres of the giant planets and their moons. The key molecules were methane and ammonia, and the electromagnetic radiation driving the chemistry was the Lyman alpha solar emission line in the vacuum ultraviolet at 121.6 nm. It was Okabe and his NBS colleagues who provided much of the hard science in support of this objective. The table had been set for the writing of this scholarly book by the appearance in 1964 of a review of the field of vacuum ultraviolet (VUV) photochemistry by McNesby and Okabe [2]. The long term impact of Okabe’s work is illustrated by Nobel Laureate Roald Hoffmann’s devoting a chapter in his 1995 book [3] to Okabe’s 1961 paper on the photolysis of ethane [4] (“a text book case for the application of the scientific method”). Another Nobel prize winner, Yuan T. Lee, published a paper in 1999 [5] with his colleagues in which testimony is given that Okabe was the discoverer of molecular elimination of hydrogen in VUV photolysis of hydrocarbons. The Lee paper was the first in which molecular elimination from propane was studied using very sophisticated techniques not even imagined at the time of Okabe’s 1961 discovery. Lee and coworkers used 157 nm laser excitation of propane and employed photofragment translational spectroscopy to measure directly the site-specific molecular elimination of hydrogen. They determined that the elimination of molecular hydrogen from the central carbon atom was the dominant process. This is what Okabe concluded 37 years earlier on the basis of experiments with deuterated propanes [6,2]. As a result of the book and his series of research papers, Okabe became widely known as a leader in the field of vacuum ultraviolet photochemistry. There have been more than 600 (non-self) citations to Photochemistry of Small Molecules, and it continues to be regarded as the definitive text on this subject. Prior to Okabe’s work at NBS (which extended from 1959 to 1983), little attention was given to photochemical requirements for measurements in the vacuum ultraviolet, such as spectral purity of light sources, appropriate window materials, and the effect of temperature on the transmittance of these materials. When he first joined the photochemistry group at NBS, he was given the choice of a project to pursue, but was urged to consider photochemistry in the vacuum ultraviolet and soon made this commitment. He quickly recognized the necessity of establishing the purity and monochromaticity of his light sources. Early attempts to study VUV photochemistry with electroded resonance lamps were plagued by outgassed impurities which made it impossible to know the active wavelengths in the photolysis. Okabe’s meticulous methods produced the first high