The Ford scientists' study of the reactxon of oxygen atoms with acetaldehyde yielded a parallel mechanism to the hydrogen atom reaction for the first two steps. The oxygen atoms abstract hydrogen atoms from acetaldehyde to form first the acetyl radical, CH3CO, and then ketene. But in this case, the OH radical is formed rather than molecular hydrogen. The rate constant of the first step of the reaction is 1.8 X 10 - 1 3 cc. molecule -1 sec. - 1 This is about five times faster than the first step of the reaction of hydrogen atoms with acetaldehyde. The subsequent reactions are more complex because more free radicals are produced as intermediates. The first step in the reaction of hydrogen sulfide with oxygen atoms gives two free radicals, HS and OH, Dr. Weinstock and Dr. Niki have found. The rate constant for this reaction is 4.9 X 10 13 cc. molecule -1 sec. -1 , they calculate. These free radicals react further to form, as the major end products, molecular hydrogen, sulfur dioxide, and sulfur. However, the reactions are complex and still not understood quantitatively. Nevertheless, the Ford pair can follow the formation rates of hydrogen atoms, molecular hydrogen, sulfur monoxide, and sulfur dioxide, all of which are present in comparable amounts. Many similarities exist between the Ford group's results and those of an earlier study on the hydrogen sulfideoxygen atom reaction by G. Liuti, S. Dondes, and Dr. Paul Harteck of Rensselaer (N.Y.) Polytechnic Institute. However, the RPI study was made under different experimental conditions, and a quantitative correlation of the two studies is not possible at the present time, Dr. Weinstock says. Oxygen atoms react with chlorine gas in two steps, Dr. Weinstock and Dr. Niki have found. In the first step, an oxygen atom reacts with Cl 2 to give OC1 and CI. In the second step, an oxygen atom reacts with OC1 to give O2 plus a chlorine atom. The results confirm a mechanism proposed by earlier workers. The Ford scientists determined the rate constant of the first step by measuring the decay of chlorine in a system rich in oxygen atoms. The rate constant was 7.5 X 10 - 1 4 cc. molecule -1 sec. - 1 at room temperature. This represents the first direct measurement of the rate of the first step, the Ford scientists say. The Ford pair also observed the OC1 radical directly in the mass spectrum. The radical's concentration remained relatively constant, independent of the extent of the reaction, they found. They estimated the second reaction's rate to be greater than 10 - 1 1
cc. molecule -1 sec. - 1 This agrees with an earlier estimate by M. A. A. Clyne and J. A. Coxon of the University of East Anglia in the U.K. In working with this system in Dr. R. G. W. Norrish's laboratory at Cambridge University, England, Dr. Frederick Kaufman of the University of Pittsburgh noticed that when a small amount of Cl 2 was added to a stream of oxygen atoms, the chlorine acted as a catalyst and was responsible for the recombination of a 10-fold excess of oxygen atoms into molecular oxygen. Dr. Kaufman reasoned that a chain mechanism was probably involved in which one of the original free radicals was regenerated. However, when the Ford scientists followed the change in concentration of chlorine directly, they found that this catalytic effect was the result of the recombination of chlorine atoms on the reactor walls. When they treated the walls in the usual manner with hydrogen fluoride, they found that after a few milliseconds the decay in the Cl 2 concentration leveled off. This probably occurred because of the regeneration of Cl 2 by chlorine atom recombination. However, when the walls of the reactor were poisoned for chlorine atom recombination with phosphoric acid, this effect was absent, and the first order decay of Cl 2 continued regularly.
Anthracene conducts via autoionization
154TH
ACS NATIONAL MEETING Physical Chemistry
A better understanding of how light creates electric current carriers in anthracene crystals is stemming from research by Dr. Martin Pope and his coworkers at New York University's Washington Square chemistry department. Using an apparatus similar to the one Dr. Robert A. Millikan used to determine the charge on the electron, Dr. Pope and his associates have concluded that simultaneous production of positive- and negative-charge carriers in anthracene crystals caused by the absorption of light of sufficient energy can be described as an autoionization process. The excitation energy produces a transitory superexcited, bound molecular state (autoionizing state) that can either ionize or spontaneously decay back to the ground state, Dr. Pope explains. Compounds such as anthracene and naphthalene are scientifically important as they were among the first organic compounds used as scintillators to detect high-energy radiation. To
select or modify materials for more sophisticated high-energy particle counters, however, more knowledge of the mechanisms by which the incident energy is degraded and transformed into light is necessary, Dr. Pope points out. Using an organic compound such as anthracene as an energy transfer model can also be important in understanding energy transfer in a biologically significant material such as chlorophyll, Dr. Pope adds. Study of anthracene, for example, can provide insight into photosynthesis mechanisms and, perhaps, the electrical behavior of biological membranes, he says. Dr. Pope's experimental setup to determine the energy levels of organic crystals is based on a technique devised over 50 years ago by Dr. Robert A. Millikan to measure the charge of the electron. A negatively charged crystal is dropped through a hole in the center of the top of two parallel plates. The crystal is suspended in space by adjusting the voltage between the two plates. After the crystal is balanced, light is focused on the crystal, and the energy, or frequency of the light, is gradually increased. At a certain energy, the crystal begins to fall, indicating that it has lost some negative charge. This loss is caused by the emission of electrons, and the energy level at which the emission occurs is termed the external photoelectric threshold of the substance. Dr. Pope's technique can also be used to determine the internal photoelectric threshold, or band gap. Charge carriers (holes and electrons) may be produced in anthracene by deposition of sufficient energy. They may result from a direct process involving a single energetic particle, or from an indirect process involving the cooperative participation of two or more particles of lower energy. The role played by excitons is an example of the indirect process. An exciton is an electronically excited molecular state that can travel through matter by energy transfer. It has sometimes been likened by scientists to a hole-electron pair that resides on the same molecule. The positive hole is left in the ground state when an electron is excited to an upper energy level. A singlet exciton is thus produced when a light photon striking an anthracene molecule is energetic enough to excite an electron to the first excited singlet state, Dr. Pope explains. In the brief interval before the electron recombines with the "hole" it left in the ground state, the hole-electron pair (the singlet exciton) can travel as a unit through the crystal from one molecule to another. From 10,SEPT. 25, 1967 C&EN
43
MODIFIED CHAMBER. Dr. Martin Pope (standing) and Jose Burgos of New York University's Washington Square chemistry department perform an experiment to determine energy levels of anthracene crystals. The apparatus is a modified version of that used by Dr. Robert A. Millikan 50 years ago to measure the charge of an electron. Study of anthracene can provide insight into photosynthesis mechanisms and, perhaps, the electrical behaviors of biological membranes
electrons. Dr. Pope has thus termed this exciton state a charge-transfer (CT) exciton. A CT exciton, in which the electron and hole are on different molecules, can be produced when light strikes a molecule in the ground state and excites an electron to an energy level in a conductive band. In the absence of an electric field the electron will fall back toward the ground state. But it will stop at the charge-transfer level for a short time. During this interval the electron and hole are bound together by their electrical attraction and move through the crystal as a pair. Two such CT excitons can collide and annihilate each other, adding their energies to promote an electron to a level high enough to send it completely out of the crystal. From their experiments with the Millikan condenser technique and with the use of polarized light to generate free carriers, Dr. Pope and his NYU coworkers, Dr. Nicholas Geacintov and Jose Burgos, conclude that a bound molecular state (autoionization state) in crystalline anthracene is a kinetic intermediate in the intrinsic formation of free carriers—at least for energies up to 6.3 e.v. The same autoionization (AI) state or superexcited state, as it is sometimes called, can result from the absorption of a single energetic photon or from the collision of two excitons: •hv
(6.3 e.v.) + A - > [A**]^»A+ + er
000 to 100,000 such transfers can take place before the exciton disappears. An exciton can participate in the creation of charge carriers in an anthracene crystal by several different mechanisms. In one case the exciton strikes the surface of the crystal where the crystal is in contact with the anode. The exciton ejects its excited electron into the anode, leaving behind the positive hole, which can move through the crystal toward the cathode. Another mechanism comes into play when the crystal is separated from the electrodes by a layer of some impurity such as oxygen. When the exciton enters the surface region of the crystal, it gives up its electron to a molecule of oxygen, again freeing the hole to travel toward the cathode. A third mechanism for generating charge carriers does not involve actual contact with the surface of the crystal. Two excitons can collide and annihilate each other, giving off enough energy to ionize the molecule at the site of the collision. If the collision takes place far inside the crystal, the electron will rise to an energy level high above the conduction band of the 44 C&EN SEPT. 25, 1967
crystal before falling back into the conduction band, where it can begin to move toward the anode. This process is called internal ionization. If the collision takes place close to the surface of the crystal, the electron will leave the crystal entirely. This process is called external ionization. In both processes, the holes remaining in the ground state of the molecules are free to move in the direction of the cathode when an electric field is present. The exciton can also be dissociated into a hole and electron by absorption of a photon. This can be studied thoroughly now that intense light sources such as lasers are available. Dr. Pope and his associates have obtained strong evidence of the existence of an unusual exciton state in anthracene, tetracene, perylene, and naphthalene. This new type of exciton state is produced by the recombination of holes and electrons. It can't be produced by direct absorption of a single photon of equivalent energy to the exciton state. The efficiency of producing this new exciton state closely parallels the efficiency of producing free holes and
• A* (3.15 e.v.) + A* (3.15 e.v.)^> [A**]^> A+ + eThe first equation represents the absorption of an anthracene molecule (A) by a single photon of energy (6.3 e.v.). The second equation represents the annihilation of two anthracene excited singlet excitons (A*), each of energy 3.15 e.v., to produce the intermediate AI state (A**). The term /?' is the rate constant for the exciton-exciton interaction process producing the intermediate state (A**), and y is the rate constant for the spontaneous dissociation of (A**). In the AI hypothesis, y is the same for both equations. With this assumption, it's possible to calculate a definite value for /?' by comparing the external electron emission currents in anthracene as produced by a single-photon process and an exciton-exciton process. The calculated value for /?' agrees well with several recently measured values for /?'. This gives considerable weight to the validity of the AI hypothesis. Furthermore, according to the AI hypothesis, the efficiency of generat-
ing free carriers intrinsically, rj, should be independent of the absorption coefficient k of the light used. This expectation was realized in studies by Dr. Pope and Dr. Geacintov. They found that rf was independent of the angle of polarization of the absorbed light, 0. By contrast, k was a strong function of $. Dr. Pope and his coworkers also found that impurities in the surface region play an important role in determining important surface reactions. For example, they found that if crystals of a compound such as anthracene are exposed to air, even in the dark, oxygen diffuses into the surface region to a distance of 100 to 200 A. This impurity concentration reaches a mole fraction approaching 10 - 4 and quenches excitons that arrive in this region. Thus, the lifetime of a singlet exciton in the surface region of anthracene that has been exposed to air is about 5 X 10 - 9 second compared with a lifetime of about 2 X 10" 8 second in the interior of the crystal, Dr. Pope says. The NYU group came to this conclusion by adding a known exciton quencher, tetracene, to the anthracene crystal. They found that the surface concentration of excitons was not affected by tetracene until the tetracene concentration exceeded a certain amount. Adding tetracene molecules represented a type of titration. The end point represented the point where the tetracene concentration was equal to the impurity concentration in the surface region. Observations of earlier workers seemed to indicate that the energy required to produce free holes and electrons in the bulk of an anthracene crystal, Egy was less than that required to excite the first singlet state in this material, Es. But the NYU group has shown that Eg is greater than E8 and that the apparent ability of the singlet excitons to produce a free carrier— which would imply that Es is greater than Eg—is due to the dissociation of the exciton by the electrode or by an impurity near the surface. The recombination of a hole with an electron would be expected to be kinetically second order, Dr. Pope says. However, there is good evidence that at least a sizable portion of the recombination process is first order (and not pseudo first order). In anthracene this results from two properties. One is a large Coulomb capture radius of an electron by a hole in this low-dielectric-constant medium. The other is the small mean free path of an electron in its normal conduction band. Thus, an electron that has received enough energy to leave its parent molecule will often fail to escape from the capture radius unless it
receives additional energy from an external electric field. The electron will therefore fall back into the same molecule that it started from, producing the observed kinetic behavior for the recombination process. This is also called geminate recombination. From the AI mechanism, Dr. Pope predicts that by decreasing the vibrational frequencies in the molecule, he can increase the ionization efficiency of the molecule in the crystal state. One recent experiment performed at Stanford University by Dr. W. E. Spicer and Dr. Barry Schechtman indicates that chlorinated phthalocyanine has a higher normalized efficiency of ionization than normal hydrogenic phthalocyanine. This seems to bear out Dr. Pope's prediction. The prevalence of autoionization and geminate recombination processes with low ionizing efficiencies also helps explain why x-rays produce such low yields of free electrons and holes.
G.e.v. protons induce m.e.v. product trend
154 TH
ACS NATIONAL MEETING Nuclear Chemistry and Technology
Brookhaven National Laboratory scientists have found that the bombardment of uranium with very-high-energy protons results in fission processes characteristic of the deposition of only tens of millions of electron volts of
energy in the uranium nucleus. This behavior, according to BNL's Dr. Gerhart Friedlander, was indicated when they found that the yields of neutron-excess products in the rareearth region—masses from 143 to 161 —from bombarding uranium with 28G.e.v. protons, fall off sharply with increasing mass the same as did yields with low-energy interactions. Dr. Friedlander explains that the knowledge of the distribution of reaction yields by mass and atomic number is a prerequisite for any detailed understanding of nuclear reactions induced in uranium and other highatomic-number elements by protons in the G.e.v. energy range. He points out that this new research confirms an earlier BNL finding that neutron excess products of multi-G.e.v. fission result from processes involving only relatively small amounts of deposition energy (about 50 m.e.v.) in a uranium nucleus. In earlier work the BNL group had found that at proton energies of 1 to 28 G.e.v., the distribution of product yields along isobaric chains in the mass region 130 to 140 shows a doublehumped structure with peaks both on the neutron-excess and neutron-deficient side of beta stability. In the present work, Dr. Friedlander says, the double-humped structure in the charge dispersion curve—first noted for products with masses under 140—not only persists but is more pronounced with products where the mass number (A) is greater than 140. The peak-to-valley ratio of the
RARE-EARTH SEPARATION. Elna-Mai Franz of Brookhaven National Laboratory loads a rare-earth sample onto an ion exchange column. The method is used to separate rare earths formed by irradiating uranium-238 with high-energy protons SEPT. 25, 1967 C&EN 45
