Photoemission from adenine crystal: Solvation of a pre-ionizing state

CHEMICAL PHYSICS LETTERS

Volume 165, number 4

19 January 1990

PHOTOEMISSION FROM ADENINE CRYSTAL: SOLVATION OF A PRE-IONIZING STATE Marek ZIELINSlU ChemistryDepartment

I, Martin POPE, Ning WANG, Csaba HORVATH ’ and Nicholas E. GEACINTOV and Radiation andsolid

State Laboratory, Mew York University. New York. NY 10003. USA

Received 16 October I989

Photoemission studies of adenme microcrystals in the presence of traces of water yield two distinct values of the ionization energy, 1,=6.2 and 7. I eV, each of which has been observed separately in different laboratories. We ascribe these results respectively to photoemission from “wet” and “dry” sites on the crystal surface. The 0.9 eV lowering of 1, is attributed to the solvation of the pre-ionizing state. The solvation energy must be in place before the electron leaves and it is argued that the photoemission event takes about 5 x IO- I3s.

1. Introduction The potential for purely electronic as opposed to ionic long range charge transport in DNA and proteins is a question that has attracted interest for almost fifty years [ 11. A central question in this matter is the role of water. It is known that the presence of water in DNA and protein samples increases the conductivity and decreases the activation energy for conductivity [ 21. The water could be affecting the carrier generation process, the carrier mobility, or both. The conductivity could be due to the movement of electrons or ions, or both. Although there is evidence for electronic conductivity enhancement in the presence of water [ 31, the concomitant increase of ionic (essentially protonic) conductivity introduces an element of doubt into the final interpretation. In addition, the effect of water on both types of carriers must be considered. By studying the effect of water on the photoemission process, it is possible to obtain information about the generation mechanism for the free electron in the conduction process. In the photoemission process the threshold energy

required to produce photoemission depends markedly on the degree to which the hole that is left behind in the valence band following the departure of the electron, is polarized within the time of the photoemission event. Since photoemission occurs in the sub-picosecond time range, the ability of water to act as polarizing medium is severely challenged. In this paper we present the results of photoemission studies on crystals of adenine. Such studies have been carried out in the past; however, the reported values of measured ionization energy (I,) differ greatly. The value of Z, = 6.4 & 0.15 eV was obtained by Subertova et al. [ 5 1, while Pong and Inouye [ 61 have reported I,= 7.0 + 0.3 eV. We have solved this ten-year old problem because with our technique we were able to reproduce both values simultaneously, simply by varying the degree of exposure of the crystal to water vapor. In our experiments, WC show that the presence of adsorbed water on the surface of adenine crystal is sufficient to lower the ionization energy by about 0.9 eV. Since photoemission is a purely electronic phenomenon, this result applies solely to the effect of water on the energy required to produce a free electron in adenine.

’ Present address: Clairol Research Laboratory, Stamford, CT 06922, USA. ’ Mason Laboratory, Department of Chemical Engineering, Yale University, New Haven, CT 06520, USA. 0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland )

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2. Experimental procedures The Millikan-Pope-Arnold (MPA) photoemission technique has been described in detail previously [ 71. The experiments were carried out in the improved version of the apparatus, capable of measuring photoemission in the vacuum UV region. In the MPA apparatus a small crystal of the studied maafter being charged terial ( z 10 pm in diameter), triboelectrically, is injected and suspended electrically between parallel horizontal plates of the capacitor, as was done by Millikan in his oil-drop experiment. The experiments wcrc performed in an atmosphere of high purity nitrogen (99.999%): in some stages of the experiment a controlled water vapor pressure was maintained, after which the system was pumped down to lo-” Torr prior to admission of nitrogen. The measured variable is the voltage required to suspend the particle, which is related to the particle charge by rhe Millikan relationship, ( v/h=w,

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The widely used method of representing the experimental data is to plot Y’j3 versus E,; the intercept of the straight line segment with the abscissa gives the value of ionization energy I,. The proportionality constant A in eq. (3) is a function of the geometry of the particle, the intensity of the absorbed light, and the concentration of the species that is photoemitting.

4. Results and discussion Typical results of the photoemission measurements for adeninc arc presented in fig. 1. The most striking feature of this plot is the appearance of the two straight line segments, instead of the usual one. Such a phenomenon has been observed before [9]

(1)

where q is the particle charge, m is its mass, d is the disiance between the plates and g is acceleration due to gravity. From ( 1) one obtains i=dy/dr=!ngdd(

l/V)/dt,

(2)

where z is the photoemission current which, normalized with respect to the incident light intensity gives the photoemission quantum yield Y in electrons per incident photon. The samples of adenine were “purissimum” grade obtained from Fluka Company, further purified by sixfold recrystallization from water. The crystals contained no water of hydration. This was determined by direct analysis using the Fisher technique; to the limit of sensitivity of the method (1 part in 105) the samples were free of water.

3. The photoemission quantum yield For a large number of organic solids a cubic dependence of the quantum yield on the photon energy E,, is observed near the threshold for photoemission

181 Y=A(Ep-Ic)3, 298

(3)

6

6.5

7

7.5

8

8.5

9

Photon energy [ev]

Fig. 1. Photoemission yield of crystalline adenine plotted as Y”‘versus photon energy; (a) adenine dried for 24 h in vacuum at 110°C (A); (b) crystal exposed to water vapor for 30 min prior to pumpingdown to 10e6 Torr (m). The high energy portions of both curves corrected for low energy, as described in the text, are plotted as well for the dried ( + ) and humidified ( x ) sample. The lower segments of both lines mtercept the abscissa at the ionization energyof the “wet” crystal, I,,, while the intercept of the corrected higher energy segments yield the “dry” adenine ionization energy, I_+

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in the studies of vacuum photoemission in polydiacetylene. The effects there were ascribed to surface heterogeneity due to the simultaneous existence of monomer and polymer. In the present situation, surface heterogeneity is effected by partial (and reversible) coverage with adsorbed water. We will assume that there are two types of emitting surface sites. One site will be considered “wet” (w ), the other “dry” (d ). If one assumes, that photoemission can occur from these two different sites having different ionization energies I,, and Icd, then the total photoemission yield is Y=UE,

-L)‘+&(+U3

>

(4)

where A, and A, are the relative contributions of the two sites to the total photoemission yield. In view of eq. (4), Y’13 is in general no longer a linear function of EP. However, assuming Icd>Ic,.,, for IO< E,< Icd we have Y= A, (E, -I,,) 3 and one can determine both I,,,, and A, from the intercept and the slope of the lower section of the plot. One can then plot [ Y-A,(E, -ZcW)3] “3 versus E, and determine the second pair of parameters I,, and A,. Such calculation, performed for dried and humidified adenine crystals, yielded generally the same values of Zcdand I,,, while A, and A, depended on the sample humidity. We have obtained the following values of ionization energies for crystalline adenine: Icw = 6.23 kO.07 eV and I,, = 7.1 kO.2 eV. These are averages for 15 samples. The relative lengths of the two straight line segments changed with the degree of exposure to water vapor. Thus, after 24 h drying of pure crystalline adenine at 110°C in vacuum, the segment that extrapolated to lower ionization energy diminished in extent, leading to a shift of the knee in the curve to lower energies. Subsequently, water vapor was introduced to the chamber for 30 min before the chamber was pumped down to 10P6 Torr and tilled with dry nitrogen. The photoemission plot changed drastically, the higher segment of the line almost completely disappearing. This process was fully reversible so that both curves (a) and (b ) (fig. 1) were obtained on the same sample. Chemical analysis of the dried as well as the humidified adenine samples proved incapable of detecting water in the crystal lattice. This result can be rationalized by the hypothesis that all the water is bound to surface molecules.

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1990

If one assumes that some of the surface is covered by water molecules and the rest is not, then the ratio A,/(A, +Ad) =p is the fractional water coverage. In the case of the dried adenine we have p= 3.45% and for wet sample p= 11%. (One should keep in mind, that after exposure to water vapor, the “wet” samples were pumped to high vacuum ( 10v6 Torr) before the photoemission measurements.) The red-shift of the ionization energy of dry adenine in the presence of a partial coverage of water molecules may have important implications with respect to the dielectric relaxation time around an upper excited state. In order for the ionization energy of adenine (w) to drop by 0.9 eV from that of adenine (d), the polarization energy gained by the hole (cation) that is left behind after photoemission from adenine (w) must be present before the electron leaves the crystal. A possible microscopic description of the ionization process for an adenine molecule that is in contact with a water cluster first involves the formation of a highly excited vibronic state upon the absorption of a photon. Assume that at t= 0, there is no polarization energy due to water that is available to the surface molecule. If the photon energy is greater than 6.2 eV but less than 7.1 eV, the excited adenine molecule will therefore not have enough energy for one of its electrons to leave the surface of the crystal. However, the adenine molecule would be in an extended electronic state. This extended state could be a Rydberg state which could either autoionize or decay back into the ground state, but no spectroscopic evidence exists as yet for such a state in adenine. If such a state existed, then assuming a hydrogenic model, the radius of a Rydberg state whose energy is 0.9 eV below the continuum would be about 7.5 A. The radius of a cluster of six water molecules is about 2 A. (According to Jot-tner and Gaathon [ IO], a minimum of six water molecules is required to bind an electron. ) This Rydberg state would have a radius large enough to include the water molecules of the cluster, and these water molecules would be required to orient and translate around the positive core of the adenine molecule during the lifetime of the Rydberg state. A solvated Rydberg state was suggested by Muto et al. [ 111 to explain the red-shift in the fluorescence of aliphatic amines when they are dissolved in various solvents; the absorption spectra in these sol299

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vents were unaffected by the solvent, indicating that no ground state interactions were involved. The stabilization energy of the solvated Rydberg state in triethylamine dissolved in relatively non-polar ether was calculated to be 0.7 eV. The longitudinal relaxation time rL that describes the polarization time of water [ 121 at 25°C is z6x lo-i3 s which is presumably comparable to the lifetime of the Rydberg state. A similar conclusion with regard to the rate of solvation was reached by Ottolenghi, and by GoldSchmidt and Stein [131 who concluded that photoionization of an aromatic molecule in a polar liquid takes place before the excited molecule can relax to some lower-lying singlet state. It is, however, not necessary to invoke a large radius Rydberg state to rationalize the involvement of six or more water molecules around the excited adenine molecule. According to Lee and Lipsky [ 141 who studied the photoionization of tetramethyl-pphenylenediamine dissolved in tetramethylsilane, the large spatial extent of the excited electron need only penetrate slightly into the solvent; the molecular core would begin to polarize the solvent, thereby increasing the electron penetration, which in turn results in an increased polarization energy. With this boot strap mechanism, thcrc is a time delay while the full polarization energy develops. This time delay results in a frequently observed decreased ionization efficiency relative to the internal conversion. Most of the polarization energy derives from relatively few molecules in the near neighborhood of the ionizing molecule [ 15 1. Therefore, if a surface adenine molecule is surrounded by water molecules on one side, and by adenine molecules on the other, the effective dielectric environment of this surface molecule would be dominated by the contribution of the water molecules, which have a higher dielectric constant. The importance of the top layer of surface molecules was demonstrated by Duke [ 161 who measured a difference of 0.3 eV in an ionization potential between first and second layers of molecules in an anthracene crystal. The high frequency dielectric constant D, of water is 5.5 [ 121 while that of adenine is probably less than about half of this. The photoemission threshold energy of molecular adenine is calculated to be about 8.5 eV [ 171 while that of crystalline adenine is measured at 7.1 eV. The POlarization energy for the hole in dry adenine is there300

19January 1990

fore about 1.4 eV. Given the increased dielectric constant of the water layer, it is therefore not surprising that the polarization energy of the hole that is partially surrounded by water molecules gains an additional 0.9 eV totalling 2.3 eV. This would result in a photoemission threshold of 6.2 eV. Another possibility for lowering the photoemission threshold is that purely on a random basis, there is a population of water clusters in which all the molecules are oriented with their dipoles pointing towards the positive core of a newly created excited adenine molecule. In this case, it would not be necessary for the water molecules to move during the photoemission process. This mechanism cannot be ruled out. However, there may be reason to doubt that it is the dominant process. Assume that each adenine site is surrounded by a cluster of six molecules. The probability that a cluster has all its molecules pointing in a particular direction is 2-6, if the molecule is restricted to only two directional choices. The concentration of favorable clusters is therefore 2-6 per site. Thus, one would find that only about 2% of the surface sites are required to be able to lower the threshold for photoemission. As a matter of fact, as may be calculated from fig. 1, the effective water coverage varied from about 3% to 11%. However, in the cluster calculation it was assumed that the entire surface was covered by water, and this is not the case because the breakpoint in fig. I shifted with changes in the partial pressure of water indicating incomplete coverage. Furthermore, the water molecules can adopt more than two configurations, so the probability that all the molecules in a cluster are oriented properly is much less than 2-6. A point of great interest is the time scale of the solvation of the pre-ionizing state. The polarization energy of 0.9 eV could possibly be attributed entirely to the vibronic contribution of the solvent, which sets in in times < IO-l4 s [ 161. The photoionization event in aromatic hydrocarbons is known to be competitive with internal conversion [ 15 J, which takes place typically in times of IO-i3 s. Thus, if the 0.9 eV could be provided by a vibronic relaxation, then the solvation would be essentially instantaneous and no nuclear motion of the solvent would be necessary. However, according to Delahay and Dziedzic [ 18 ] who studied photoemission from ions in aqueous solution, the vibronic contribution to the polarization

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PHYSICS LETTERS

energy of an ion induced by a change of valence of 1, is 0.2 eV. This is contributed by the inner sphere solvation. If the outer sphere solvation is included, then at a photon energy of 7 eV, the total contribution is 0.3 eV. It thus appears that in order to gain the full polarization of 0.9 eV, there must be some nuclear motion, and this has been calculated to occur in times of about 0.5 ps [ 121. The photoemission event must therefore occur in that time frame, or slower. The present work which involves purely electronic phenomena, provides support for the conclusions of Rosenberg [ 193, who attributed the lowering of the activation energy of conductivity of proteins upon hydration to the increase in the polaritability of the hydrated protein.

This work was supported by the National Foundation for Cancer Research and by the US Department of Energy Grant DEFG0286-ER60405. Acknowledgement is made of useful conversations with C.B. Duke, S. Lipsky, C.E. Swenberg and L. Christophorou.

1990

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Science 93 ( 1941) 609. [ 21 M.R. Powell and B. Rosenberg, Biopolymers 9 ( 1970) 1403. [ 31 R. Pethig, Dielectric and electronic properties of bIologica materials (Wiley, New York, 1979). [4] M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Oxford Univ. Press, Oxford, 1982) p. 459. [ 51 C.E. Subertova, J. Bok, P. Rihak, V. Prosser and E. Silinsh, Phys. Stat. Sol. (a) 18 (1973) 741. [6] W. Pong and C.S. Inouye, J. Appl. Phys. 47 ( 1976) 3444. [7] L. Altwegg, M. Pope, S. Arnold, Wm.Y. Fowlkes and M.A. El Hamamsy, Rev. Sci. Instr. 53 ( 1982) 323. [8] M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Oxford Univ. Press, Oxford, 1982) p. 543. [9] A.A. Murasov, E. Silinsh and H. Blssler, Chem. Phys. Letters 93 ( 1982) 148.

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