J . Phys. Chem. 1987, 91, 4999-5001
4999
LETTERS Low-Energy (0-1 0 eV) Electron Transmission Spectra of Multilayer Tryptophan Films G. Leclerc, T. Goulet, P. Cloutier, J.-P. Jay-Gerin,* and L. Sanche Groupe du Conseil de Recherches MPdicales du Canada en Sciences des Radiations et DZpartement de MZdecine NuclPaire et de Radiobiologie. Facult2 de MZdecine, Universiti de Sherbrooke, Sherbrooke, QuZbec, Canada J1 H 5N4 (Received: May 1 1 , 1987; I n Final Form: July 21, 1987)
We describe an experimental method for measuring the transmission of slow ( < l o eV) electrons through thin solid bioorganic films at 300 K, with application to the amino acid tryptophan. The data were analyzed with the aid of a simulation model of electron transport in condensed media, which allowed, for the first time, an estimation of low-energy electron scattering mean free paths in a biological solid.
Introduction Absorption of ionizing radiation gives rise in matter to a large number of electrons of a few tens of electronvolts. The quantitative knowledge of the transport properties of these slow electrons is thus a basic prerequisite in radiation chemistry and biology for a better characterization of the energy degradation processes in irradiated media, as well as for evaluating the yield, nature, and spatial distribution of chemically active species formed during the early stages of radiation action.' In recent years, low-energy (510-20 eV) electron transmission (LEET) spectroscopy experiments on thin solid films deposited on a metal surface have been successfully used for studying the behavior of slow electrons in condensed molecular systems.24 In particular, these experiments have proved to be of considerable value for assessing basic transport parameters, such as elastic and inelastic electron scattering mean free paths (MF'P). In this Letter, we report on the first LEET experiments performed on a biological solid at 300 K, with application to the amino acid tryptophan. Electron scattering MFP in this material are estimated from the analysis of the data with a simulation model of electron transport in condensed matter and on the basis of a novel method for measuring film thicknesses.
modified the standard LEET experiment to allow the investigation of biomolecules, such as tryptophan, which are solid at room temperature and reduced pressure. The experimental apparatus used here is shown schematically in Figure 1. Electrons emitted from a filament are aligned by an axial magnetic field of 50 G and are energy-discriminated by passing through a trochoidal monochromat~r.~The electron beam leaves the monochromator with an energy spread of 0.15 eV and an intensity Io of about 3 nA (f5%).) Its diameter (-1 mm) is constant over the energy range investigated (0-10 eV). A moleculaT source is used to deposit the films on a thin polycrystalline platinum foil (99.95% purity) held at room temperature. It consists in a pierced stainless steel cavity containing the sample (- 1 mg of powder). The cavity is indirectly heated by a tantalum filament. In order to deposit films of uniform thicknesses, the source is located at about 9 cm from the platinum substrate. A mask limits the angular spread of the molecular beam and hence contamination of the target region. The target has an open area of 1 cm2 and is mounted on the axis of a rotating feedthrough to direct it toward either the molecular source or the electron beam. The platinum substrate is cleaned by resistive heating at 1000 OC. Prior to film growth, the sample is baked under vacuum at 50-100 OC for 3 h to remove water vapor. The whole apparatus is housed in a sorption- and ion-pumped ultrahigh-vacuum (UHV) system with a base pressure of Torr in the absence of a sample and Torr when a sample is present in the source. of about 2 X D,L-Tryptophan was obtained from Matheson Coleman and Bell and used without further purification. Under the UHV conditions of the experiment, tryptophan molecules could be evaporated at temperatures between 150 and 200 OC without being pyrolyzed. In fact, chromatographic studies indicated that sublimation increased the purity of the tryptophan sample from 97% to 99.57L6 Moreover, film degradation by electron beam irradiation was found to be negligible for the short period of time (typically, -10 s) needed to record a LEET spectrum. X-ray and high-energy electron diffraction studies were also performed on the deposited films and showed no evidence of long-range crystalline order. Assuming, therefore, that the films are amorphous, no structural order7 or interference* effects are expected to be seen in the LEET spectra reported herein.
Experimental Section The LEET experiment has been described in detail elsewhere.2 Its essential features can be summarized as follows. A wellcollimated monochromatic electron beam is incident normally from vacuum on a thin solid film grown in situ on a metallic surface from the vapor of the studied molecules. Plots of the electron current Ittransmitted through the film to the metal are recorded at different film thicknesses as a function of the incident electron energy E (relative to the vacuum level). In this work, we have ( I ) Platzman, R. L. Radiat. Res. 1955, 2, I . See also: Ore, A. In Effects of Ionizing Radiation in DNA: Physical, Chemical and Biological Aspects; Hiittermann, J., Kohnlein, W., Thule, R., Bertinchamps, A. J., Eds.; Springer: Berlin, 1978; Chapter 2, p 21. Proceedings of the Workshop on the Interface between Radiation Chemistry and Radiation Physics; Dillon, M. A,, Hanrahan, R. J., Holroyd, R., Kim, Y.-K., Sauer, Jr., M. C., Toburen, L. H., Eds.; Argonne National Laboratory: Argonne, IL, 1983; ANL-82-88. (2) Sanche, L. J . Chem. Phys. 1979, 71, 4860. Sanche, L.; Perluzzo, G.; Michaud, M. J Chem. Phys. 1985, 83, 3837. (3) Bader, G.; Perluzzo, G.; Caron, L.-G.; Sanche, L. Phys. Reu. B: Condens. Matter 1982, 26, 6019. Plenkiewicz, B.; Plenkiewicz, P.; Perluzzo, G.; Jay-Gerin, J.-P. Phys. Reu. E Condens. Matter 1985, 32, 1253. Caron, L.-G.; Perluzzo, G.; Bader, G.;Sanche, L. Phys. Rev. E: Condens. Matter 1986, 33, 3027. Keszei, E.; Jay-Gerin, J.-P.; Perluzzo, G.; Sanche, L. J . Chem. Phys. 1986,85, 7396. Goulet, T.; Keszei, E.; Jay-Gerin, J.-P. In Proceedings of the Sixth Tihany Symposium on Radiation Chemistry, BalatonszZplak, Hungary, 1986; AkadBmiai Kiado: Budapest, in press. (4) Goulet, T.; Jay-Gerin, J.-P. Radiat. Phys. Chem. 1986, 27, 229.
( 5 ) Stamatovic, A,; Schulz, G. J. Reu. Sci. Instrum. 1970, 41, 423. (6) For details on the high-performance liquid chromatographic (HPLC)
procedure used in this work, see: Langlois, R.; Ali, H.; Brasseur, N.; Wagner, J. R.; van Lier, J. E. Photochem. Photobiol. 1986, 44, 1 1 7. (7) Bader, G.; Perluzzo, G.; Caron, L.-G.; Sanche, L. Phys. Reu. E: Condens. Matter 1984, 30, 78. See also: Leclerc, G., unpublished work. (8) Perluzzo, G.; Bader, G.; Caron, L.-G.; Sanche, L. Phys. Reu. Lett. 1985, 55, 545.
0022-3654187 , ,1209 1-4999$01.50/0 0 1987 American Chemical Societv I
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5000 The Journal of Physical Chemistry, Vol. 91, No. 19, 1987 r
Letters
,
to electrometer
SIMPLE HOLDER,
*
EVAPORATION SOURCE
/ TARGET
U
MONOCHROMATOR
Figure 1. Schematic diagram of the experimental apparatus (see text).
The total number of molecules deposited on the metal substrate was determined by dissolving the film in triply distilled water and by measuring the ultraviolet (UV) light absorbance of the solution with a Varian 2200 spectrophotometer. The sensitivity of the technique permits the investigation of biomolecules such as the base constituents of nucleic acids and the amino acids which are all soluble in water at room temperature for the very low concentrations considered here (-50 nM). From the knowledge of the total number of molecules and of the density of the adsorbed thin solid film, we were able to deduce its average thickness with 15% uncertainty. Since the film densities are generally unknown, tabulated values for the bulk solid could be used; however, for the sake of accuracy, we have measured the density of the films using a method recently developed by Leclerc et al.9 This method consists essentially in measuring spectrophotometrically the UV absorbance of a multilayer film dissolved in water, in combination with the measurement of its thickness using a light interference technique. For our tryptophan films, we estimated the density to be 1.28 (f0.06) g/cm3, which corresponds to a monolayer film thickness of about 6.4 A. We also found that they were free of aggregates and that the variation of their thickness was less than 5% over the entire surface.
Results and Analysis Figure 2a shows the LEET spectra I J I 0 vs. E for electron impact on a clean platinum substrate without film and with about 9-, 14-, and 16-A coverages of solid tryptophan at 300 K. As can be seen, the deposition of a tryptophan film changes the work function of the platinum substrate3 and leads to an energy shift in the onset of the transmitted current of about 0.2 eV. The reported electron energies E are defined with respect to this new potential of the vacuum level. The broad maximum observed in the Z,/Io vs. E curves around 6 eV is due to the first three important electronic transitions of tryptophan at 4.4, 6.1, and 7.5 eV.'oJ' These transitions, identified with vertical bars in Figure 2a, constitute energy-loss channels for the excess electron and contribute to increase the transmitted current due to a reduction in the probability that the electron returns to v a c ~ u m . ~ - ~ The knowledge of the onset of the first inelastic electronic excitation E,, and the first exciton energy E, allows an estimation of the energy Voof the bottom of the film's conduction band with respect to the vacuum level.' For tryptophan, E , is 4.0 eV93I0while the analysis of LEET spectra shows that the inelastic threshold occurs at 3.3 eV.' We thus find Vo= Eit- E , N -0.7 eV, which indicates that the conduction-band minimum of solid tryptophan lies below the vacuum level. From the known correlation of electron mobility with V0,l2such a significantly negative Vovalue (9) Leclerc, G.; Goulet, T ; Cloutier, P.: Jay-Gerin. J.-P.; Sanche, L Can J . Phys., in press. (10) Lin, S . D. Radiat. Res. 1974, 59. 521. Wetlaufer. D. B. In Aduances in Protein Chemistry; Anfinsen, Jr., C. B., Anson, M. L., Bailey, K., Edsall, J. T., Eds.; Academic: New York, 1962; Vol. 17, p 303. (1 I ) The energies of these transitions were also determined in this work
by measuring the negative value of the second energy derivative of the transmitted current, -d2/,/dE2 (see ref 9). (12) See, for example: Cheng, I.-Y.; Funabashi, K. In Proceedings of the Fifth International Congress of Radiation Research, Seattle, WA, 1974; Nygaard, 0 . F., Adler, H. I., Sinclair, W. K.. Eds.; Academic: New York, 1975; p 415.
I II 0
i d0 i . . . , , . . O M ELECTRON ENERGY ( e V )
Figure 2. Comparison between experimental and calculated LEET spectra: (a) Experimental f t / l o vs. E curves obtained for electron impact on a clean polycrystalline platinum substrate without film and with 9-, 14-, and 16-A coverages of solid tryptophan at a temperature of 300 K.
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The vertical bars represent the energies of the first three important electronic (optically allowed ?r ?r*) transitions of tryptophan (ref IO). (b) Electron transmitted spectra as calculated from our model for solid tryptophan film thicknesses of 2 and 3 times the electron scattering mean free path I and for three values of P,,, (0.03, 0.04, and 0.05). Pvlbwas chosen equal to 0.2 (ref 16).
suggests that excess electrons may exhibit high mobility in this material. In order to determine the electron elastic and inelastic scattering MFP, we used a theoretical electron transport model4 based on a semiclassical description of electron dynamics in s01,ids.'~ This model consists in a semianalytical simulation of the transport of an excess (Bloch) electron in the film's conduction band combined with a random sampling of the temporal succession of the various elastic and inelastic scattering events. The injected electron is assumed to be scattered isotropically by internal imperfections (including phonons, stacking faults, and impurities) and to transfer its energy to the medium through the excitation of intramolecular vibrations and of electronic transitions. For the sake of simplicity, in the energy range 0-10 eV considered here, the total electron scattering MFP 1 is taken to be independent of E, and the effective mass of the electron is chosen as equal to the free-electron mass4 Using this simplified model, the analysis of the data consists in adjusting the values of the elastic and inelastic MFP in the simulation until an agreement is found between the theoretical and experimental LEET spectra. In our calculations, we define the probability P, that the process "n" occurs at the time of a scattering event as the ratio of the total MFP 1 to the M F P I, corresponding to this process (P, = I / l n ) . The energy losses considered are the three well-defined electronic transitions at 4.4, 6.1, and 7.5 eVl0 and one typical vibrational excitation of 0.25 eV.14 The ionization threshold for solid tryptophan being around 8 eV,ISionization processes wefe neglected for E < 10 eV. Since no information is available on slow-electron scattering by solid tryptophan, we were forced to make certain simplifications regarding the energy dependence of the scattering probabilities. of vibrational exFirst, we considered that the probability Pvib citation does not vary between 0 and 10 eV. Second, we assumed that the three electronic transitions are in direct competition and that the sum Pel=of their probabilities is anstant for all incident electron energies between 4.4 and 10 eV. Inside this energy range, the probabilities of these electronic energy losses AE are considered to behave similarly; Le., they are equal to zero at threshold, they increase linearly until they reach a maximum value of Pelecat E (13) See, for example: Ashcroft, N. W.; Mermin, N. D. Solid S a f e Physics; Holt, Rinehart and Winston: New York, 1976; p 213. (14) Koegel, R. J.; Greenstein, J. P.; Winitz, M.; Birnbaum, S. M.; McCallum, R. A. J . Am. Chem. So?. 1955, 77, 5708. Duval, C. Mikrochim. Acta 1957, 326. ( 1 5 ) Cannington, P. H.; Ham, N. S. J . Electron Spectrosc. Relal. Phenom. 1979, 15, 1 9 .
J . Phys. Chem. 1987, 91, 5001-5003 = AE, and they decrease linearly when the next energy loss probability starts to rise.9 With such simplifying assumptions, we cannot expect our calculated LEET spectra to mimic perfectly the experimental ones. However, as seen in Figure 2, the general shape of the measured LEET spectra for film thicknesses of 14 and 16 8, (Figure 2a) is well-reproduced by the theoretical curves (Figure 2b). A comparison of the experimental and theoretical results suggests that 15 f 2 A is between 2 and 3 times I and that Pel,,is within the range 0.03-0.05. This gives 4.3 A C 1 C 8.5 A 90 A C lele,C 280 8, In analogy with the scattering of electrons in dilute gases, we can define the electron scattering cross section per molecule for the process “n” as (16) The value of Pvlb= 0.2 was chosen to reproduce the shape of the experimental transmission curves between 0 and 1 eV. This choice was found not to influence greatly the transmission values for incident electron energies greater than 3 eV.
500 1 u,
= 1/ N I ,
where N is the number density. Thus, the total and electronic excitation scattering cross sections in solid tryptophan are 3.1
X
cm2 < utOtC 6.1
X
cm2
0.9
X
cm2 < uelec< 3.1 X
cm2
In conclusion, we have shown that LEET experiments can provide a powerful means for estimating electron scattering MFP and cross sections in a biological solid. It seems perfectly feasible to apply the same procedure to other bioorganic molecules and thus to extend our knowledge of the interaction of slow electrons with the constituents of biological media.
Acknowledgment. We thank P. BEnard, R. Wagner, and J. Sapieha for valuable assistance. This work was supported by the Conseil de Recherches Midicales du Canada, by the Fonds de la Recherche en Santi du Quibec, and by the MinistZre de 1’Enseignement Supirieur et de la Science du QuCbec. This support is herewith gratefully acknowledged.
Remeasurement of N(2P) 4- O2 Reaction Rate Using Multiphoton Ionization Detection of Nitrogen Atoms Charles M. Phillips,* Jeffrey I. Steinfeld, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39
and Steven M. Miller Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Massachusetts 01 731 (Receiued: June 15, 1987)
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We have observed photoionization signals in a flowing afterglow reactor arising from the (2 + 1) photoionization of N(2P) and N(2D), resonantly enhanced by the two-photon transitions N[3p 2P:] N[2p3 zPyJ and N[3p 2Sy,2] N[2p3 ’OS], respectively. A diffusion-limited lifetime of 530 ws at 0.22 Torr was observed for N(2P), indicating wall quenching with unit efficiency. Measurement of N(2P) quenching by O2yielded a rate coefficient of (1.8 & 0.2) X cm3 s-l, which is significantly less than values previously obtained by using vacuum-UV resonance detection of the metastable nitrogen.
Introduction The creation of vibrationally excited nitric oxide through the reaction of nitrogen atoms and molecular oxygen has been the subject of considerable interest due to the importance of N O as an infrared emitter in the upper atmosphere. Above 150 km in the quiescent atmosphere, the reaction N(4S) + O2 NO(u17) + O(3P) AHo = -1.38 eV (1) is a significant contributor to the production of NO(u>l). Several groups14 have investigated this reaction, obtaining the vibrational branching ratio for the energetically allowed levels of the product NO. Under auroral conditions, the metastable nitrogen atoms N(2P) and N(’D) are the important contributors to the formation of N O in the upper atmosphere. These react with molecular oxygen [reactions 2a-2eI approximately 4 orders of magnitude faster than
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(1) Winkler, I. C . ;Stachnik, R. A.; Steinfeld, J. I.; Miller, S. M. J . Chem. Phys. 1986, 85, 890. (2) Whitson, Jr., M. E.; Darnton, L. A.; McNeal, R. J. Chem. Phys. Lett. 1916, 41, 552. (3) Rahbee, A.; Gibson, J. J . Chem. Phys. 1981, 74, 5143. (4) H e m , R. R.; Sullivan, B. J.; Whitson, Jr., M. E. J. Chem. Phys. 1983,
79. 2221.
0022-3654/87/2091-5001$01.50/0
do ground-state nitrogen atoms [reaction 11.5-10
+ -
N(2D) + O2
NO(u518)
+ O(3P) AH’ = -3.77 eV (2a)
+ O(lD) AH0 NO(u126) + O(3P)
NO(u18)
N(2P)
O2
+ O(lD) NO(u53) + O(lS)
NO(u114)
= -1.80 eV
(2b)
AHo = -4.96 eV (2c) AHo = -2.99 eV AHo = -0.77 eV
(2d) (2e)
Product state distributions of NO(u) for reactions of metastable nitrogen atoms with oxygen have been reported for 1 ,< u ,< 12 by Kennealy et a1.l’ using infrared chemiluminescence (IRCL). ( 5 ) Lin. C. L.: Kaufman. F. J . Chem. Phvs. 1971. 55. 3760. (6) Slanger, T. G.; Wood,’B. J.; Black, G. Geophys. Res. 1971, 76, 8430.
i.
(7) Husain, D.; Kirsch, L. J.; Wiesenfeld, J. R. Faraday Discuss. Chem. SOC.1972. 53. 201.
(8) Husain; D.; Mitra, S. K.; Young, A. N. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1721.
(9)Young, R. A,; Dunn, 0. J. J Chem. Phys. 1975, 63, 1150. (10) Iannuzzi, M. P.; Kaufman, F. J. Chem. Phys. 1980, 73, 4701.
0 1987 American Chemical Society
