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values, however, the decrease in R , due to increased quantum efficiency (q2 = 1.0) is not substantial. In Figure 8 we show the dependence of the photochemical enhancement factor on the incident power, for KF = 10" s-l, q2 = 0.01, and I,,, = lo3, lo8, and 1O'O W/m2. Increased power diminishes the enhancement factor, essentially because in vacuum the process is proportional to the power. The slight decrease of the enhancement is caused by the fact that the stimulated emission from level 111) is enhanced by the presence of the sphere and this increases the radiative energy loss from level 111). So while the absolute photochemical rate goes up with power, the enhancement by the sphere goes down. For high powers one must also be concerned with the effects of heating and multiphoton processes. In concluding we emphasize that we have considered here only the quenching caused by energy transfer, as given by local elecI I trodynamics. For metallic substrates the nonlocal effects30 are 01 I short ranged and are important only at the distances at which the 30 Bo 90 120 150 enhancement factors are very low. Therefore, unless one is d ti, --c specifically interested in monolayers or bilayers the nonlocal effects Figure 8. The enhancement factor (R,) for the photochemical rate as The parameters u = 200 &jZl can be disregarded. Another short-range quenching mechanism a function of distance d for various Iinc. is the charge transfer from the excited molecule to the = 0.1, w = wzl = 3.48 eV, rz= lo9 s-l, and KF = 10" s-' are used for This seems to be the predominant mechanism33whenever the work all the curves. For curve (a), Iinc= lo3 W/m2; for curve (b), Iinc = lo8 W/mZ; for curve (c), Ii,, = W/mZ. function of the surface exceeds the ionization potential of the excited state by less than34approximately 1 eV. We have made low and high quantum efficiencies, respectively. The various no attempt to include this effect here. curves are for various values of K F . The enhancement of slow rates is fairly marginal at all distances, essentially because surface Acknowledgment. We benefited substantially from discussions quenching of state 111) is more efficient than its depletion by the with Abe Nitzan. The work was supported in part by the Office photochemical process. For the curve c (Figure 7a), having the of Naval Research and by N S F Grant CHE82-06130. highest value of Kpc,the quenching process prevails at low distances d , but is overcome at roughly 20 A. The reason for this is that the enhancement factor 11 + R(w)I2decays with the distance slower (33) F. Bozso, J. T. Yates, Jr., J. Arias, H. Metiu, and R. M. Martin, J . Chem. Phys., 78,4256 (1983); F. Bozso, C. P. Hanrahan, J. Arias, J. T. Yates, than the quenching rate; therefore at some intermediate distance Jr., H. Metiu, and R. M. Martin, Surf.Sci. Lea,128, 197 (1983); F. Bozso, it prevails and yields an enhancement factor of -50. Increasing J. Arias, C. Hanrahan, R. M. Martin, J. T. Yates, Jr., and H. Metiu, Surf. the quantum efficiency of the molecule (Figure 7B) substantially Sci., 136, 257 (1984). suppresses the enhancement a t slower rate of K p . At larger K p (34) S.Sawada, A. Nitzan, and H. Metiu, Phys. Reu. B, in press.
'
Surface Ionization of Some Basic Organics on an Oxidized Rhenium Emitter. Thermionic Emission in a Protonated Form Toshihiro Fujii,* Haruhiko Suzuki, and Masahiro Obuchi Division of Chemistry and Physics, National Institute for Environmental Studies, Tsukuba, Ibaraki 305, Japan (Received: February 8, 1985; In Final Form: July 1 , 1985)
The formation of ions under surface ionization conditions has been studied mass spectrometrically for basic organic molecules. Mass spectra are discussed with respect to kinds of ionic species, their principal mechanism of formation, and energetic considerations. The surface ionization involving the catalytic effect of the emitter surface, as well as the surface reaction between adsorbed species, is responsible for most of these ions. The efficiency of the formation of the protonated molecules on the hot oxidized Re emitter surface seems to depend on the energy of ion formation (appearance potential, the predicted AP) which is given from the calculation using the literature value of the proton affinity (PA) of the molecule M and the bond dissociation energy D [(M-H)-HI on the assumption of the postulated bimolecular reaction process. The current density of the (M + H)' ions is qualitatively correlated with the predicted AP.
Introduction In a previous publication by the one of the authors,' the detailed investigation of the (M+H)+ formation on the oxidized Re surface has been described for pyridine molecules. It was found that the bimolecular surface reaction of the proton-transfer complex formation, followed by the ion desorption, is the most probable mechanism for the intense ions of pyridine compounds. The result of this study, together with the knowledge of ion-molecule reaction *To whom correspondence should be addressed.
0022-3654/85/2089-4687$01.50/0
in the gas phase,2 gives rise to the speculation that desorption of protonated molecules may take place for other organic compounds of high proton affinities3v4from the hot surface under the conditions that the acid sites exist on the emitter ~ u r f a c e . ~ (1) Fujii, T. J . Phys. Chem. 1984, 88, 5228. (2) Yamdagni, R.; Kebarle, P. J . A m . Chem. SOC.1973, 95, 3507. (3) Walder, R.; Franklin, J. L. Int. J. Muss Specfrom.Ion Phys. 1980, 36, 85.
(4) Arnett, E. M.; Jones, F. M.; Taagepera, M.; Henderson, W. G.; Holtz, D.; Beauchamp, J. L. J . A m . Chem. SOC.1972, 94, 4724.
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Fujii et al.
TABLE I: Tested Organics (M), Their SI Ion Species, and Predicted Appearance Potential (AP) for Formation of Their Protonated Molecules by Two Types of Reactionsg AP 2M (M + H)+ M +N (M + M (IP) obsd SI ions PA d (M-1) e H)' + (N-1) e pyridine (9.27) (M+H)+ 9.7' 8.0 6.98 aniline (7.70) M', (M+H)' 9.3" 3.5 7.8 7.38 dimethylamine (8.24) (M-H)*, (M+H)' 9.76 4.1 8.0 6.98 y-picoline (9.04) (M+H)' 9.9' 3.82 7.52 6.78 N-methylaniline (7.3) (M-H)', (M+H)' 9.6d 3.2 7.2 7.08 tert-butyl alcohol (9.97) 8.6' 4.6 9.6 8.08
+
-
+
-
+
'Taft, R. W.; et al. J. Am. Chem. SOC.1973, 95, 381 1 . bHenderson, W. G.; et al. J . Am. Chem. SOC.1972, 94, 4728. CAue,D. H.; et ai. J . Am. Chem. SOC.1976, 98, 854. dBriggs, J. P.; et al. J . Am. Chem. SOC.1972, 94, 5128. 'Beauchamp, J. L.; et al. J . Am. Chem. SOC.1972, 94, 2638. fAll the values from ref 10. PIP, ionization potential; PA, proton affinity; D,bond dissociation energy. All the values in eV. "This species is observed only in the case of M + N.
It would be of interest to find out whether this mechanism holds in general; however, this idea has not been investigated. The use of energetic considerations may be one approach to this question. The concept of appearance potential (AP)6 for the energy analysis of ions is particularly useful. The AP is commonly defined as the minimum energy which is required to form the ion concerned. When this definition is employed in the present study, the A P is obtained from thermodynamic literature data for the postulated mechanism of ion formation. As a possible process involved in protonation of the organic compound, M, one can suppose that process is shown in eq 1, in which two identical molecules on the surface produce the ion of interest: 2M
-
(M+H)'
+ (M-H) + e
(1)
For the calculation of the AP of (M+H)+, we assume the following hypothetical reaction mechanism without any involvement of chemical interaction with the surface. M (M-H) + H (2)
-
-
H-H++e M AP(MH')
+H
(3)
(M+H)'
= D[(M-H)-HI
+ IP(H) - PA(M)
(4)
(5)
where D is the bond dissociation energy of reaction 2, IP(H) is the ionization potential of hydrogen atom, and PA(M) is the proton affinity of M. A second possibility is the reaction between the organic molecule M and other species N as a proton donor. M
+N
-
(M+H)+
AP(MH') = D[(N-H)-HI
+ (N-H) + e + IP(H) - PA(M)
(6) (7)
A similar reaction may also occur, involving the surface M
+ H,O
-
(M+H)+
+ OH* + e
(8) when the remaining O H radical may be bound to the active site (*) of the emitter surface. The A P for the process via reaction 8 decreases as the O H bonding energy at the surface increases. The purpose of the present study is to deduce general rules which govern what kinds of organic molecules generate (M+H)+ ions under certain surface ionization (SI) conditions and to what extent the energy of ion formation may be correlated to ion intensity. It is also interesting to determine the relationship of the relative amounts of the protonated ions formed by processes 1 and 6 to the difference of the dissociation energy of the bond whose cleavage yields the atomic hydrogen. For these purposes, the mass spectrometric experiments have been conducted using the oxidized Re emitter surfaces for several basic organic compounds. All the resulting mass peaks and intensities are discussed in terms of the energetic considerations (5) Basila, M. R.; Kantner, T. R.; Rhee, K. H. J . Phys. Chem. 1964,68, 3197. (6) Levsen, K. "Fundamental Aspects of Organic Mass Spectrometry"; Weinheim: New York, 1978; pp 108-113.
based upon the predicted AP. Simple organic compounds were chosen because of the possibility of obtaining the necessary data which are used for the calculation of the AP's.
Experimental Section All work was performed on a Finnigan Model 3300 quadrupole mass spectrometer equipped with the home-built thermionic ion source. The Re emitter was placed in the manufacturer's E1 ion source chamber. The instrumental details and operational procedure have been described elsewhere.',' The constant admission of sample organic gas was accomplished by using a pressure gauge (capacitance manometer, MKS 3 15 BHS-10) and a molecular flow element (MKS, FE 1.0) that can be used as a control unit (1 70-44A).8 For the dual admission of samples, the additional gas was admitted to the mass spectrometer from the other reservoir via a variable leak valve (Series 203, Granville-Phillips). The sample gas pressure was measured with an ionization gauge (VG-1, Wakaida), which was calibrated beforehand for all the used compounds against another capacitance manometer (3 15 BHS-L, MKS). The S I spectra of six organic compounds were taken. For each compound, M, a set of two experiments was conducted in terms of the sample combination: the S I spectrum of only M and the SI spectrum of a M/cyclohexene mixture. All the nonlabeled chemicals (reagent grade) and the deuterated water of CEA product were purchased from a chemical company (Nakarai, Tsukuba) and used without further purification. The ultimate vacuum in the analyzer tube measured by the ion gauge after bakeout was typically in the 10-8-torr range. The oxidized Re emitter filament was prepared and used in the torr which was presence of O2at a controlled pressure of 2 X obtained by adjusting the Granville-Phillips variable leak valve (Series 203). Results A plot of the signal of protonated molecules ( i ) against the emitter temperature (7') showed almost the same bell shape for all the tested compounds as that for the pyridine molecules.' The temperature dependence is complicated for the ion formation by the proposed mechanism that the (M+H) ions are formed through the dissociative process. The dissociative reaction is the essential step, which must be the activated step. However, molecular association such as the proton-transfer-complex formation is reduced by the thermal energy at higher surface temperature. The variation of the lifetime of the molecule on the surface with the temperature is also rate-determining for the protonated molecule formation. Predicted AP's. Table I summarizes the observed S I ions as well as the thermochemical p r ~ p e r t i e sfor ~ ~six ' ~ organic compounds (7) Fujii, T. Int. J. Mass Spectrom. Ion Processes 1984, 57, 63. (8) Kiesling, R. A.; Sullivan, J. J.; Santeler, D. J. J . Vac. Sci. Techno). 1978, 15, 771. (9) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. ReJ Data 1977, 6. (10) Weast, R. C., Ed. "60th CRC Handbook of Chemistry and Physics"; CRC Press: Boca Raton, FL, 1979-1980; pp F-231-F-239.
Surface Ionization of Some Basic Organics studied. The A P s for the formation of the (M+H)+ ions via the two types of reaction processes described by eq 1 and 6 were determined. The calculations were made on the assumption that any adsorbed species are physically bound to the emitter surface and any adsorption energy can be ignored. For the reaction with N (eq 6), cyclohexene was chosen because of its low bond dissociation energy. The A P of the protonated molecule formed through the reaction of M + M lies below the IP of the sample molecule for all the compounds except aniline. Pyridine. The S I spectrum of pyridine has already been discussed in detail elsewhere: only the (M+H)+ ion appears in the S I spectrum of pyridine.'J1 The experiments using the deuterated compounds showed that (M+H)+ could be attributed to the bimolecular reactions involved with the proton transfer on the surface which are expressed as eq 1, 6, and 8. Presumably, certain kinds of organic gases remaining in the analyzer play the role of N in eq 6. The proton transfer takes place in a concerted step in which pyridinium ions form through the surface reaction between N compound and pyridine molecules. The acidic properties of the surface of oxidized Re emitter are different from those of a pure Re metal. The S I spectra of pyridine obtained with oxidized Re and pure Re emitters under the same conditions showed a significant difference in the (M+H)+ intensity; consequently, the Brmsted acid properties of the surface contribute to the (M+H)+ ion formation. In the present study, a SI spectrum was taken for the mixture of pyridine and cyclohexene. It showed a much greater intensity of the (M+H)+ ion than that of a pyridine/D20 mixture under the same conditions of the sample size and component ratio. The choice of cyclohexene was based upon the prediction that the pyrolytic dissociative reaction which gives rise to hydrogen atom occurs favorably for the weaker bond [D(C6HII-H)] = 3.08 eV and D(H-OH) = 5.2 eV] and hence leads to the intense pyridinium ion formation. Aniline. For the admission of pure aniline, only the molecular ion is observed at the higher surface temperature. The result is in agreement with Zandberg's result.I2 The formation of molecular ions of aniline can be interpreted as a Saha-Langmuir model surface i~nization'~ as a result of the low IP of aniline (7.70 eV) on the high work function surface of the oxidized Re emitter.I4 The increase in ion intensity with the rise of temperature also indicates this ionization model. The absence of (M+H)+ in the spectrum is consistent with the fact that the AP for the anilinium ion is bigger than the ionization energy of the aniline molecule. In the aniline/cyclohexene mixture, however, the mass spectrum consists of a low-intensity protonated molecule at lower emitter temperatures instead of molecular ions. This is to be expected because the energy for the formation of (M+H)+ is considerably less than the I P of the aniline molecule. Dimethylamine. The S I of dimethylamine was also measured with two sample procedures. Both the spectra exhibit the strong (M-H)+ ion ( m / e 44) and much weaker protonated molecule ( m / e 46) around the emitter temperature of 900 O C . The m / e 44 ions are formed preferentially with increasing emitter temperature. It is w e l l - k n o ~ nthat ~ ~ the ~ ~ formation of (M-H)+ ions from many organics results from surface ionization which involves the (M-H) radical formation of surface reaction products followed by the Saha-Langmuir model surface ionization. M (M-H) + H (9)
-
+
(M-H) (M-H)+ e (10) Reaction 10 is the secondary step after the establishment of thermal and charge equilibrium between the (M-H) radical (1 1) Zandberg, E. Ya.; Rasulev, U. Kh. Sou. Phys.-Dokl. (Engl. Transl.) 1970. 14, 169. (12) Zandberg, E. Ya.; Rasulev. U. Kh. Sou. Phys.-Tech. Phys. (Engl. Transl.) 1969, 13, 1450. ( 1 3) Zandberg, E. Ya.; Ionov, N. I. "Surface Ionization", Israel Program for Scientific Translation, Jerusalem, 1971. (14) Greaves, W.; Stickney, R. E. Sur/. Sci. 1968, 11, 395. (15) Davis, W. D. Emiron. Sci. Res. 1978, 13, 395.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4689
'I
c
.=
I
Na+
4690
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
transfer reactions are observed under field ionization conditions when the proton affinities of M and (M-H) satisfy the following condition: PA(M) - PA(M-H) L 0 (11) This relationship can be transformed as the following equationi3 D[(M-H)-H] IP(H) - PA(M) I D[(M-H)-HI IP(H) - PA(M-H) (12)
+
+
AP(MH+) IIP(M) The results of the present SI measurements demonstrate that eq 13 should hold in the surface ionization. If it is assumed that surface interaction is not involved at all, the requirement for the protonated molecule formation on the emitter is that the energy necessary for formation of protonated molecule (AP) must not be larger than the ionization potential of the molecule. The (M+H)+ peak and absence of M+ in the spectrum of y-picoline and pyridine indicate that energies equal to the ionization potentials of these compounds are not supplied during proton-transfer reactions and that the ionization potential of the molecule M does not necessarily need to be supplied in order to form the protonated products. Thus, if the AP's of two ionic species differ, with AP(1) < AP(2), then Z[AP(l)] > I[AP(2)] can be expected, where Z represents the ion intensity of the respective ions. This relationship is valid for the protonation process of the present study and qualitatively explains both the intensity distribution in the spectrum and the relative intensity of the protonated ions among the tested organics. The observation of the small (M+H) ion peak of dimethylamine could be an indication that the use of the oxidized Re filament for surface ionization allows the occurrence of protonation process with AP as high as 8.0 eV. But it should be limited to the conditions that (M+H)+ ion is more stable than any other ion species such as M+ and (M-H)'. Dissociative Surface Zunizatiun. The intensity of the (M-H)+ ion is most abundant in the SI spectrum of dimethylamine and N-methylaniline. One of the possible mechanisms is the concerted process
M AP[(M-H)']
-
+H +e = IP(M-H) + D[(M-H)-HI (M-H)+
(14) (15)
Fujii et al. N-methylaniline and dimethylamine, respectively. Even if the effect of adsorption energy of atomic hydrogen is taken into account, these values are too high to allow the formation of the ions. The consecutive process is more likely: the formation of surface dissociative reaction product with a low IP followed by the Saha-Langmuir model surface ionization on the hot surface emitter with a high work function. In this mechanism, the ionization currents for (M-H) radical are determined not only by the ionization efficiency (it is given by the Saha-Langmuir equation on the assumption of the establishment of thermal and charge equilibrium between the species on the surface material and the surface material) but also by the efficiency of the formation of the (M-H) product on the surface.' The (M-H)+ ion peaks of dimethylamine and N-methylaniline can be explained by this process since the ionization potentials of N-methylanilino radical and (CHJ2N radical are sufficiently low to be ionized,'.'* 7.8 and 6.0 eV, respectively.
Conclusion The mechanistic, energetic, and kinetic study of the protonated molecule formation on the hot Re surface has been made using six basic organic molecules. The (M+H)+ ion formation process competes with the M+ ion formation process and the consecutive Saha-Langmuir model SI of (M-H)+. The ionization probability among these processes seems to be dependent on the energy of formation of each ionic species. The AP of the (M+H)+ ion of the postulated biomolecular proton-transfer reactions can be calculated with the thermodynamic data. This predicted AP is used to determine which ionic species appear preferentially in the SI mass spectrum. The present study also suggests that (M+H)+ formation reaction on the catalytic oxidized Re surface is approximately adiabatic; the IP of the molecule does not necessarily need to be supplied for the (M+H)+ ion formation. However, this study does not give a comprehensive explanation for the observation of (M+H)+ ions under the present surface ionization conditions, because the feasibility of surface ionization of a molecule during thermal desorption of a sample does not depend on only a low appearance potential but on some other conditions which exclude a pure thermodynamic treatment. Further studies still remain for better understanding.
According to the calculations, AP should be 10.1 and 11.O eV for
Acknowledgment. We are grateful to Dr. H. Soma for helpful discussions.
(16) Rollgen, F. W.; Beckey, H. D. Z . Naturforsch., A: Phys., Phys. Chem., Kosmophys. 1974, 29A, 230.
Registry No. Re, 7440-15-5; pyridine, 110-86-1; aniline, 62-53-3; dimethylamine, 124-40-3; y-picoline, 108-89-4; A'-methylaniline, 10061-8; tert-butyl alcohol, 75-65-0.
