SCIENCE
Extreme Vacuum Ultraviolet Light Source Developed Frequency doubling of laser light in a crystal, followed by frequency tripling in puffs of gas provides tunable light near 100-nm wavelength The first tunable source of coherent, extreme vacuum ultraviolet (XUV) radiation has been developed by chemical researchers using only commercially available, relatively inexpensive components. The system will allow a variety of experiments to be conducted that were possible previously only at synchrotron radiation sources. XUV radiation is that segment of the electromagnetic spectrum with wavelengths shorter than 104 nm and longer than about 10 nm, the wavelength that begins the soft x-ray region of the spectrum. The upper limit of 104 nm is determined by what is known as the lithium fluoride cutoff. Lithium fluoride crystals transmit radiation longer than 104 nm, but no known solid transmits the shorter wavelength radiation of the XUV region. Because of this cutoff, techniques such as frequency doubling in a nonlinear crystal, which have been used to produce coherent radiation in other portions of the spectrum, cannot be applied to produce XUV light. Chemistry professor Richard N. Zare and coworkers Charles T. Rettner and Ernesto E. Marinero of Stanford University, Palo Alto, Calif., and Andrew H. Kung, managing scientist of the San Francisco Laser Center, developed the method, which converts visible light into UV radiation. Their technique can be used even in the XUV region. It is an extension of an idea originally de28
February 7, 1983 C&EN
veloped by Kung. The research was supported by the National Science Foundation and the Office of Naval Research. The researchers use a standard, pumped dye laser. Used separately, two dyes allow the laser to produce visible light tunable from 584 to 614 nm. This light is focused on a nonlinear crystal and emerges frequency doubled as radiation with a tunable range of 292 to 307 nm. In the frequency doubling process, a photon enters a material, in this case a crystal, and sets up electronic oscillations in the material. When a second photon of the same wavelength enters the material, it interacts with the oscillations caused by the first photon to produce a photon with half the wavelength (and twice the frequency) of the original photons. The process is nonlinear because two photons generate a single photon (C&EN, Oct. 4,1982, page 18). Until now, Zare says, one was stuck if one wanted to use this technique to produce XUV radiation, since no solid material transmits it. But using a gas at reasonably high pressure, Zare can triple the frequency of incoming light. However, to carry out experiments with the light, the gas has to be removed after the frequency change has taken place. To accomplish this, the researchers use a nozzle valve to squirt a small amount of gas into a vacuum chamber. Near the valve, the gas is at reasonably high pressure; farther away, the pressure is very low because the gas has been pumped away. The laser is operated at 10 Hz and synchronized with operation of the valve. The frequency-doubled laser radiation is focused near the valve opening and the radiation frequency is tripled by the gas to radiation with a tunable range of 97 to 102 nm.
Zare: plenty of chemical action Zare points out that the range of wavelengths produced in the system is determined only by the dyes used in the pumped dye laser. By changing the initial dye, other XUV wavelengths could be obtained. A variety of gases can be used to frequency triple. The Stanford researchers use argon because it produces a high conversion efficiency (the number of XUV photons produced compared to the number of incoming photons), and because it does not absorb in the 97- to 102-nm region. Zare estimates that a research group could set up a similar system from scratch in about two months (the time it takes for suppliers to deliver the equipment) at a cost of $50,000 to $75,000. Zare and his coworkers are using the system to study the electronic excitation of hydrogen molecules in a quantum specific manner. The hydrogen molecule is difficult to study spectroscopically. It does not possess a dipole moment, so it has no di-
EDUCATION pole-allowed infrared transitions. Infrared quadrupole transitions occur but require kilometer-long path lengths to detect and so are useless in the laboratory to study hydrogen in the gaseous state. The first electronic transitions occur in the wavelength region produced by Zare's system. The system detects hydrogen to as low as 2 X 108 molecules per cc and, with fairly simple improvements in light baffling, Zare predicts this can be improved to about 107 molecules per cc. What is important is that the detection is quantum specific. That is, a specific electronic excitation will occur at slightly different wavelengths of incident light depending on the initial vibrational and rotational state of the molecule. The laser-induced fluorescence spectrum of hydrogen molecules that Zare obtains displays a peak for each of these different initial states. The intensity of the peaks can be related directly to the population in each quantum state. The studies of molecular hydrogen have been carried out primarily to test the system. The goal is a system that will allow the researchers to study what Zare calls the most elementary reaction in chemistry: the hydrogen exchange reaction, which is simply a hydrogen atom reacting with a hydrogen molecule to produce a new hydrogen molecule and a hydrogen atom. Although it sounds pretty mundane, Zare points out that it is the reaction for which the most sophisticated theoretical ab initio calculations of reaction dynamics can be performed. Although there have been partial comparisons between those calculations and experimental data, a complete set of experimental data taken with quantum resolution on the reaction has yet to be obtained to use as a check against the calculations. Such a complete set of data will be a crucial test of the accuracy of theory, and Zare thinks that his system may be able to obtain it. The laser system, however, has uses extending far beyond that to which Zare is putting it. "This is a wavelength region where there is plenty of chemical action," Zare says. "That is why people go to synchrotron radiation sources."
Education does well in corporate giving Education claims a high proportion of corporate contributions to various activities, particularly for manufacturing companies. And chemical companies rank a bit higher than the average. This is one of the findings in a just-published analysis of corporate support of higher education in 1981 made by the Council for Financial Aid to Education, New York City. A survey of 712 companies shows a continuation of previous patterns: Although manufacturers gave almost four times as much to education as did nonmanufacturers, they gave only two and a half times as much overall to activities that also include health and welfare, culture and art, and civic activities. Corporate support of education as a whole, the council estimates, rose in
1981 by almost 11% over 1980, despite a drop in corporate profits of 4.2%. This combination of increased support and lower profits made for a record high claim by education on pretax net income of 0.49%. That proportion, CFAE says, is the highest for any year for which it has estimates, starting in 1950. CFAE uses data from the federal government. CFAE notes that the ability of corporate contributions programs to expand during periods of profit declines is due in large part to the existence of corporate foundations. Thus, CFAE's estimates include gifts and grants made by such foundations but not company payments to the foundations. On this basis, it estimates total corporate contributions in 1981 at $2.9 billion, of which $1.14 billion went to education. •
More than 40% of manufacturers' giving goes to education Million dollars, 1981
Manufacturing Chemicals (42^ $ 13,497 Electrical 15,930 machinery (45) Fabricated metals 1,509 (19) Food, beverage, 9,818 tobacco (37) Machinery (45) 5,336 Mining (8) 738 Paper, allied 3,169 products (23) Petroleum and 48,434 gas(32) Pharma4,839 ceuticals (17) Primary 4,353 metals (26) Printing and 1,666 publishing (16) Rubber (11) 1,297 Stone, clay, 1,329 glass (14) 1,294 Textiles and apparel (21) Transportation 1,997 equipment (20) Total (376) $115,208
Education Support of education support as as % of % of total PTNI contributions
Support of education
Total contributions as % of PTNI
98.8 102.9
$ 43.9 49.0
.73 .65
.33 .31
44.4 47.6
18.7
4.9
1.24
.33
26.3
70.2
23.6
.72
.24
33.6
52.5 5.8 34.2
26.0 2.7 12.0
.98 .79 1.08
.48 .37 .38
48.7 46.1 35.2
213.5
89.9
.44
.19
42.1
39.9
12.5
.82
.26
31.3
38.6
15.1
.89
.35
39.3
15.3
5.7
.92
.34
37.1
6.4 18.5
2.3 6.1
.49 1.39
.18 .46
36.0 32.8
13.1
4.0
1.01
.31
30.5
64.7
26.1
3.24
1.31
40.4
$ 793.0
$323.4
1*9
~J28
40\8
$ 323.3 $1116.3
$ 83.9 $407.3
.65 .68
.17 .25
26.0 36.4
Pretax Total net Income (PTNI) contributions
$
Nonmanufacturing Total (336) GRAND TOTAL (712)
$ 50,069 $165,277
a Number of companies reporting.
Rudy Baum, San Francisco February 7, 1983 C&EN
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