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Photoionization detection of CF2 radicals resulting from the IR multiple-photon dissociation of CF2HCl molecules in a molecular beam
Chemical Physics 271 (2001) 357±367
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Photoionization detection of CF2 radicals resulting from the IR multiple-photon dissociation of CF2HCl molecules in a molecular beam V.N. Lokhman, D.D. Ogurok, E.A. Ryabov * Institute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Moscow region, Russia Received 5 March 2001
Abstract CF ions were detected when irradiating the CF2 radicals resulting from the IR multiple-photon dissociation (MPD) of jet-cooled CF2 HCl molecules with an intense pulsed UV radiation (k 6 244 nm). The experiments performed allowed us to conclude that the CF ions observed were produced in the UV multiphoton excitation of the CF2 radicals via the ~ 1 B1 , followed by their fragmentation and ionization. This eect was used to study the resonant intermediate state A unimolecular decay of CF2 HCl under molecular beam conditions. We measured the kinetics of this process, as well as the translational energies of the CF2 radicals. The isotopic selectivity of the IR MPD of 13 CF2 HCl was found to improve substantially under jet-cooled molecular beam conditions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Multiple-photon ionization (MPI), and its resonance-enhanced version (REMPI) in particular, is being widely used to study highly excited states of molecules and radicals and also various chemical reactions involving polyatomic molecules and radicals (see, e.g., Refs. [1,2], and also the review [3]). Various [n m] REMPI detection schemes were suggested and implemented to date for a large enough number of polyatomic radicals [3]. Speci®cally, the possibility was demonstrated of the REMPI detection of a number of trihalomethyl radicals (see Ref. [3]). These radicals, as well
*
Corresponding author. Tel.: +7-95-334-0231; fax: +7-95334-0886. E-mail address: [email protected] (E.A. Ryabov).
as dihalomethylene radicals, play an important part in the photochemical processes occurring in the environment, including the upper atmosphere and stratosphere. Therefore, the availability of highly sensitive methods for detecting such radicals is important in the study of these processes. In contrast to trihalomethyls, dihalomethylenes are usually detected by the laser-induced ¯uorescence (LIF) technique. In particular, the LIF technique was used for the CF2 radical to study the kinetics of the reaction CF2 O (see Ref. [4] and references cited therein) and also the formation of this radical as a result of the IR multiple-photon dissociation (IR MPD) of CF2 HCl [5]. But the photoionization detection technique can oer a number of obvious new possibilities, as compared to LIF, especially when used in conjunction with massspectrometric analysis, including isotopic analysis possibilities. For this reason, one of the objectives
0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 4 1 7 - 7
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of the present work was to develop a highly sensitive photoionization technique for detecting dihalomethylene radicals and apply it to studies into the kinetics of reactions involving them. The radical to be studied was taken to be CF2 , and the reaction involving it, the formation of 13 CF2 and 12 CF2 as a result of the isotope-selective IR MPD of CF2 HCl, for this molecule was known to dissociate into CF2 and HCl (see, e.g., Ref. [5]). In the above-mentioned REMPI detection of trihalomethyls, use was made of two- or threephoton resonances between visible light quanta and some intermediate state in the particle of interest, a Rydberg state, as a rule (see Ref. [3]), i.e., [2 3 1] REMPI schemes. We propose to detect dihalomethylenes by means of [1 n] schemes of MPI by UV radiation via the ®rst excited singlet state. We have earlier successfully used such a [1 n] photoionization scheme to detect the radical SF5 [6]. For the CF2 radical, the ®rst singlet ~ e abstate (T0 37 226 cm 1 [7]) has its A X sorption band in the region of 250 nm [8]. It was exactly this transition that we used to implement 1 n scheme. This work has one more objective associated with the development of intense sources of radicals. A wide variety of present-day investigations require intense collimated beams of radicals. There are various methods to produce radicals (see, e.g., Ref. [3]). Most frequently used is made of pyrolytic and plasma methods. But they cannot always provide for the necessary concentration of radicals and besides, the precursor molecules in these methods may be subject to an excessive fragmentation. In the case of pulsed beams, the use of photolysis for the production of radicals seems very attractive, both UV and IR radiation being suitable for the purpose. In principle, both these approaches can supplement each other quite well. In the ®rst case, we are dealing with the dissociation of molecules via excited electronic states. The latter circumstance imposes certain requirements upon the type of the laser to be used and can restrict the choice of suitable precursor molecules. In the second case, we have to do with the IR MPD. There are a suciently large number of polyatomic molecules that have their absorption bands in the region of 9±11 lm, so that they can be dis-
sociated by means of a pulsed TEA CO2 laser ± the most common laser covering this wavelength range. The IR MPD process is very ecient: the dissociation yield can be close to unity, and what is more, it proceeds, as a rule, through the rupture of the weakest molecular bond, and so the set of the dissociation products is minimal (see Ref. [9]). Following the ®rst experiments on the IR MPD of SF6 under molecular beam conditions [10], a number of experiments were conducted to study the IR MPD of other jet-cooled molecules. A review of some of these works can be found in Ref. [9]. The analysis of the results obtained in these works shows that the use of the IR MPD process for the production of intense beams of radicals actually appears very promising. Considering everything that has been said above, our experiments on the REMPI detection of the CF2 radical and investigation of the kinetics of the IR MPD of the CF2 HCl molecules were conducted in this work under pulsed molecular beam conditions. The results obtained are presented below.
2. Experimental The experiments were conducted with an experimental set-up similar to the one described in Ref. [6]. Most of the measurements were taken under pulsed molecular beam conditions. The IR multiple-photon excitation and dissociation of the CF2 HCl molecules were carried out using a line-tunable pulsed TEA CO2 laser. To photoionize the IR MPD products, use was made of a pulsed UV radiation. The ions thus produced were detected by means of a time-of-¯ight mass spectrometer (TOF MS). The irradiation geometry is illustrated in Fig. 1. The molecular and laser beams lay in one and the same plane and intersected on the TOF MS axis (y-axis) that was perpendicular to this plane. The supersonic beam of gas-dynamically-cooled CF2 HCl molecules (a General Valve nozzle, 0.8-mm-diameter ori®ce, 200-ls pulse, fwhm) was directed into the ionization chamber through a 0.8-mm-diameter skimmer. The typical stagnation pressure was 2 atm. The molecular density distribution along the x0 - and
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Fig. 1. Illustrating the IR UV irradiation of the CF2 HCl molecules in a beam. At the top and side are the distributions of the concentration of the molecules (M) and IR radiation intensity along the x0 - and y-axis, respectively. Shown at the top is also the form of the eective FV of the TOF MS (for details, see text).
y-axis in the beam is presented in Fig. 1. It is of a trapezoidal shape with a width (fwhm) of 5 mm along the y-axis, the width of its ¯at portion being equal to 3 mm. The IR-laser radiation (pulse duration 130 ns, fwhm) was directed opposite to the molecular beam and at a small angle to its z-axis. It was focused into the vacuum chamber by means of a telescope system. The IR radiation intensity distribution in the focal plane was Gaussian to a good approximation, its diameter (at the e 1 level) being approximately equal to 0.8 mm. The length of the caustic was around 2 cm, and the laser ¯uence U0 on the beam axis could reach as high a value as 12 J/cm2 . The frequency-doubled output from a XeCllaser-pumped dye laser was used as a UV radiation source. The laser used Coumarin-47 and Coumarin-102 dyes and a BBO-crystal frequency doubler to have its UV output tunable in the range 225± 260 nm. The UV pulses (energy 120±150 lJ, duration 10 ns (fwhm)) were focused with a lens
(f 12 cm) into the vacuum chamber where they intersected the molecular beam at an angle of 60° (x0 -axis in Fig. 1). The cross-sectional area of the UV beam in the focal region amounted to some 10 5 cm2 . As noted earlier, the ions produced were detected and identi®ed by means of a TOF MS. The top curve of Fig. 1 represents the eective ®eld of vision (FV) of the TOF MS for the ionizing UV radiation geometry used. It can be seen from the distribution curves presented in Fig. 1 that the molecular beam was irradiated in its homogeneous region. The inhomogeneity of the IR radiation and the shape of the FV of the TOF MS were taken into consideration when processing the results of measurements (see elsewhere). The pulsed nozzle and the CO2 laser were operated in step so as to make the IR MPD of the molecules occur at the peak of the molecular pulse (DT ' 0:78 ms). By varying the delay time Dt between the IR and UV pulses, we could study the kinetics of the formation and subsequent evolution
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of the IR MPD products (see elsewhere). In some cases, measurements were taken with long delay times (DT ' 4:6 ms) between the molecular and CO2 laser pulses, i.e., well after the former came to an end. In this case, subject to IR UV irradiation were the particles that had repeatedly suered scattering by the chamber walls and thus reached a temperature approximately equal to the room value. The signals from the TOF MS as well as from IR and UV pulse energy detectors were monitored by a digital oscilloscope and then fed to a computer to be accumulated and processed. In our experiments, we used 99.8% pure CF2 HCl with natural concentrations of the isotopes 13 C and 12 C (1.1% and 98.9%, respectively).
3. Measurement results The IR irradiation of the CF2 HCl was eected in the 9.3-lm CO2 laser region where the symmetric m3 (1100 cm 1 ) and antisymmetric m8 (1118 cm 1 ) modes of the C±F bonds of this molecule are located [11]. The isotope shift with respect to the m3 mode comes to 24 cm 1 m3 13 CF2 HCl 1076 cm 1 , and this allowed us to tune in to both 12 Cand 13 C-containing molecules. We found that when the energy ¯uence of the CO2 laser was high enough, U0 > 3:0 J/cm2 , and the UV radiation wavelength k was shorter than 244 nm, the combined IR UV irradiation of the molecules gave rise to the CF ions, no other ionic products being detected. Fig. 2 presents the mass spectra obtained in the IR UV irradiation of CF2 HCl at the 9R 8 CO2 -laser line (1070.46 cm 1 ). The spectra were obtained with the molecular gas at room temperature (Fig. 2a) and with the gas being gas-dynamically cooled (Fig. 2b). The IR radiation frequency selected fell within the spectral pro®le of the IR MPD of 13 CF2 HCl, and so there should have predominantly occurred the dissociation of this isotopic molecule. This fully corresponds to the data of Fig. 2. Speci®cally, the 13 CF and 12 F peak intensities in Fig. 2b approximately coincide, whereas the 13 C0 =12 C0 ratio in the parent molecule amounts to 0.011. The quantitative measure of
Fig. 2. Mass spectra of the ions produced in the course of the IR UV irradiation of the CF2 HCl molecules: (a) at room temperature; (b) under gas-dynamic cooling conditions. U0 12 J/cm2 , 9R 8 CO2 laser line.
the selectivity of the process is the parameter a 13=12: a 13=12
13 CF t 13 C0 : : 12 CF t 12 C0
Under irradiation conditions of Fig. 2, the selectivity a 13=12 18 for the molecular gas at room temperature. As one would expect, in the case of gas-dynamic cooling of the molecules, the selectivity of their IR MPD process increases substantially, up to a 13=12 45:5. To enhance the CF -ion signal, all the subsequent measurements were taken at the 9R 30 CO2 -laser line (1084.64 cm 1 ) where it was 12 CF2 HCl that predominantly suered dissociation. Fig. 3 shows the dependence of the CF -ion signal on the IR radiation ¯uence. The form of this relationship is quite typical of the IR MPD process (see, e.g., Ref. [9]). At the beginning there is a region where the dependence is very strong. In our case, it is the region of U0 3 J/cm2 , which can be
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Fig. 4. CF -ion signal as a function of the UV radiation wavelength. The vertical lines indicate the position of the ~ 1 B1 e 1 A1 transitions in CF2 (according to t02 t002 0 A X the data presented in Ref. [8]). Shown in the inset is the smallscale structure in the t02 12 t002 0 transition region. U0 6:6 J/cm2 ; EUV 90 lJ. Fig. 3. CF -ion signal as a function of the IR radiation ¯uence. kUV 230:24 nm, EUV 70 lJ, Dt 0:51 ls.
interpreted as some threshold. Next follows a region where the dependence is weaker and can be approximated in some region of variation of the ¯uence U by a power function. For the data of Fig. 3, the power of this function is n 1:85. We also studied the dependence of the CF -ion signal on the wavelength and intensity of the UV radiation. Fig. 4 presents the spectral dependence of the CF -ion yield. As noted earlier, under our experimental conditions the CF ions could reliably be detected at an UV radiation wavelength of kUV 6 244 nm. As can be seen from Fig. 4, the spectral dependence obtained features a distinct structure with a period of Dm ' 500 cm 1 . In the course of our measurements, we found that the spectrum includes, in addition to this large-scale structure, also some small-scale one. A typical example of such a structure is presented in the inset in Fig. 4. The origin of these structures will be discussed in Section 4. Fig. 5 shows the dependence of the CF -ion signal on the UV radiation
Fig. 5. CF -ion signal as a function of the UV radiation energy. () room temperature; ( ) jet-cooled molecules. U0 8:5 J/cm2 , Dt 0:51 ls.
energy (intensity). The measurements were taken in the region of the maximum of the UV spectrum measured (see Fig. 4). The dependence of Fig. 5 was plotted for both the molecular beam and scattered molecular gas. In the latter case, the CF -ion signal is approximately 3.5 times as high
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as in the case of molecular beam, but the data presented in Fig. 5 were appropriately normalized. It can be seen that at EUV P 50 lJ, which corresponds to UUV ' 5 J/cm2 in the caustic of the UV beam, the yield of the CF ions rises approximately linearly with increasing UV radiation intensity. In the region of lower EUV values, this dependence is apparently stronger, this being testi®ed to by the experimental data points in hand. Unfortunately, we failed to measure this dependence more accurately, our measurement sensitivity being limited. As already noted, by varying the delay time Dt between the IR and UV pulses and changing the location of the probe UV beam, one can study the kinetics of the formation and subsequent space± time evolution of the precursor radicals of the CF ions observed. We believe (see Section 4) that the precursor of the CF ion is the CF2 radical formed as a result of the IR MPD of the CF2 HCl molecule. The pertinent relationships between the CF signal and the delay time Dt are presented in Fig. 6 for two IR radiation ¯uence values, namely, U0 6:9 and 10.8 J/cm2 . The measurements were taken with intersecting IR and UV beams, i.e., with zero displacement of the UV beam along the y-axis of Fig. 1. The leading edge of the curve obtained is associated with the formation of the CF2 radicals, and the subsequent reduction of the signal is
Fig. 7. Evolution of the spatial distribution of the CF2 radicals. Normalized CF -ion signal S Dt; y 0=S0 as a function of the distance y to the IR beam axis for various delay times Dt. The smooth curves ± model computation for vm 650 m/s. U0 10:8 J/cm2 , kUV 234:31 nm, EUV 90 lJ.
caused by their escape from the dissociation region. The latter conclusion is supported by the data of Fig. 7 which re¯ects the spatial evolution of the CF2 radicals. Presented in this ®gure are the relationships between the CF signal and the displacement of the probe UV beam along the y-axis (see Fig. 1) at various instants of time Dt. While at Dt 0:44 ls (peak in the curve of Fig. 6) the spatial distribution of the CF2 radicals does not fall beyond the limits of the CO2 laser beam, it clearly spreads wider as Dt is increased. The quantitative interpretation of these changes is presented below. 4. Discussion
Fig. 6. Normalized CF -ion signal S Dt; y 0=S0 as a function of the delay time Dt between the IR and UV pulses. (1) Shape of the CO2 laser pulse; (2) model computation; (3) model computation for the modi®ed spread function (see text). kUV 234:31 nm; EUV 90 lJ.
In our view, the results obtained allow us to draw the unambiguous conclusion that the CF ions observed are formed as a result of a series of consecutive processes, including the IR MPD of the CF2 HCl molecules and the subsequent fragmentation and ionization of the resultant CF2 radicals under the eect of the UV radiation they absorb in a multiple-photon fashion. In other words, there take place the following processes: CF2 HCl nhmIR ! CF2 HCl;
1
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CF2 mhmUV ! CF F e:
2
Indeed, the CF2 HCl molecules themselves make no contribution to the formation of the CF ions. The UV absorption band of these molecules starts in the region of 210 nm [12], which is much shorter than the UV radiation wavelength used by us. Besides, special experiments have demonstrated that the CF ions could not be observed in the absence of IR radiation. Highly vibrationally excited CF2 HCl molecules could, in principle, contribute to the formation of these ions. The IR MP excitation of molecules has been known (see, e.g., Ref. [9]) to give rise to a suciently wide vibrational distribution, up to the dissociation limit and higher. But in our case such highly excited CF2 HCl do not contribute to the formation of CF . If this had taken place, in the plot of the CF signal as a function of Dt (see Fig. 6) there would have been observed a substantial ``nonvanishing'' (in time) signal component associated with the absence of the escape of the excited molecules from the IR excitation region. Actually, as can be seen from Fig. 6, there takes place a rather rapid reduction of the signal to very low values, and the kinetics of this process is fully explained by the escape of the CF2 radicals (see later). Thus, the only precursors of the CF ions are the CF2 ions formed as a result of reaction (1). This conclusion, too, fully agrees with the character of the relationship between the CF -ion signal and the IR radiation ¯uence. As noted earlier, there occurs a yield-¯uence relationship typical of the IR MPD process. The CF signal arises only when the IR MPD threshold is surpassed. According to the data presented in Ref. [13], the magnitude of this threshold for CF2 HCl is around 1 J/cm2 . In our experiments, the CF signal arose at U0 P 3 J/cm2 . For a Gaussian beam, this corresponds to an average ¯uence of U 1:5 J/cm2 (it is exactly this averaged parameter that is usually cited in the literature), which, allowing for some dierences between the experimental conditions, agrees quite well with the above literature value. Note also that the data on the isotopic selectivity of the process (Fig. 2) also agree with the conclusion that the ®rst step in the formation of the CF ions is the IR MPD of the CF2 HCl molecules, i.e., reaction (1).
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The formation of the CF from the CF2 radicals (reaction (2)) occurs as a result of the multiplephoton absorption of UV radiation. One can estimate the energy required for this process, assuming that it is the dissociation of the CF2 radicals that is the ®rst to take place, the CF radicals thus produced being then subject to ionization. The quantity D0 CF±F 41 273 cm 1 [14], and the ionization potential of CF is I0 9:11 eV. Thus, to produce a CF ion, one has to expend no less than 11 4752 cm 1 of energy, which is slightly less than the energy of three quanta of the UV radiation used by us. In actual fact, the formation of a CF ion apparently requires more than three UV quanta. We believe that the ®rst step in such a multiphoton (multistep) process is the resonance ~ 1 B1 e 1 A1 in CF2 . The one-photon transition A X spectroscopy of this transition has been well studied [8]. Its absorption spectrum lies mainly in the range 230±280 nm and features a well-de®ned t02 t002 vibrational structure associated with the m2 bending vibration whose frequency in the upper electronic state is m02 496 cm 1 . The positions of the maxima of the pertinent bands, t02 t002 0, are shown in Fig. 4. Obviously the position of these transitions matches well enough the structure observed by us. At the same time, the transitions of Fig. 4 are perceptibly wider than their counterparts measured in Ref. [8]. Our analysis indicates that this is due to the higher vibrational and rotational temperatures of the CF2 radicals in our experiments. According to the data presented in Ref. [15], the IR MPD of the CF2 HCl molecules gives rise to suciently ``hot'' CF2 radicals with vibrational and rotational temperatures of 1160 and 2000 K, respectively. It should also be noted that on the whole the shape (envelope) of the onephoton absorption spectra presented in Ref. [8] and in Fig. 4 dier noticeably. The former has a maximum in the region of 245±248 nm, whereas the one measured by us has already terminated in this region. In our view, this agrees with the multiphoton/multistep character of reaction (2): it is exactly the subsequent transitions that determine the long-wavelength boundary of the spectrum in Fig. 4. As for the small-scale structure in aour spectrum, it is apparently associated with the rotational structure of the transitions, which
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clearly manifest itself in the one-photon absorption spectrum presented in Ref. [8]. Considering the multiphoton character of the CF ion formation process, it is dicult to interpret this structure more accurately. It is our belief that the multiphoton character of the CF -ion formation process does not contradict the character of relationship measured by us between the CF -ion signal and the UV radiation intensity (see Fig. 5). In the region of low UV radiation intensities, the nonlinear character of this relationship is clearly manifest. The subsequent passage to an almost linear dependence is associated with the saturation of the quantum transitions. Our estimates show that at least the ®rst, resonance transition is always strongly saturated, no matter what the value EUV from the range of Fig. 5. The data available on the structure of the highlying levels in CF2 are inadequate to unambiguously describe the subsequent transitions (those ~ 1 B1 e 1 A1 ) occurring following the ®rst one, A X under the eect of UV radiation. One of the possible versions is as follows. The CF2 radical has a e with an energy of T0 72 740 cm 1 [7], to state B ~ 1 B1 . If the which it can pass from its excited state A e state B is a dissociating one, the CF ion can then be formed by way of the two-photon ionization of the resultant CF radical. Other versions are possible as well, but in any case we think that most likely it is the dissociation of CF2 that is the ®rst to occur, with the ionization of CF close behind, no CF 2 ion being observed in our experiments. Let us now consider the space±time evolution of the CF signal illustrated in Figs. 6 and 7. As follows from what has been said above, it is associated with the space±time evolution of the CF2 radicals formed in the course of reaction (1). So, by measuring the signal produced by the CF ions resulting from the UV MPI of CF2 , one can study the unimolecular decay kinetics of CF2 HCl under the eect of IR radiation. The dissociation of CF2 HCl is a three-centre HCl elimination reaction, with the back reaction having a barrier. According to Ref. [16], the activation energy of direct reaction (1) is Ea 54 kcal/ mol, the back reaction barrier being DE 6 kcal/ mol. As with many relatively small molecules, the
dissociation lifetime s of CF2 HCl shortens rapidly as the excess of its vibrational energy Evib over the activation energy Ea grows higher. For example, according to the RRKM-theory calculations made in Ref. [16], when Evib exceeds Ea by 1 IR quantum, the lifetime s is around 50 ns, and it shortens down to 5 ns when the excess of Evib over Ea equals 3 IR quanta. Because of their high-dissociation rate, such molecules cannot be overexcited too much above the dissociation limit. Simple estimates show that under our experimental conditions CF2 HCl molecules can hardly be excited to a level lying above Ea by more than 3 IR quanta. Therefore, considering what has been said above, it can be taken that in the experimental conditions of Figs. 6 and 7 all the molecules had already dissociated by the moment the IR pulse ended. The leading edge of the kinetic curve in Fig. 6 is mainly determined by the duration of the IR pulse, and the subsequent reduction of the signal is caused by the escape of the CF2 radicals from the IR excitation region. By modelling this process and comparing between the theoretical and experimental results obtained, we can obtain some parameters of reaction (1). In our model, it was assumed that by the instant tp when the IR pulse came to an end all the CF2 HCl molecules had already dissociated. The initial CF2 concentration distribution measured (in the same way as the data of Fig. 7) at this moment of time is described well enough by the Gaussian function n tp ; q n0 exp q2 =R2 ;
3
where q2 x2 y 2 (see Fig. 1), and R 0:347 mm. The evolution of the spatial distribution of the concentration of the CF2 radicals was assumed to take place as a result of their free motion in accordance with their velocity distribution. We supposed that there was a Maxwell velocity distribution with a most probable velocity of vm . In our experimental geometry, it is only the transverse (normal to the z-axis) velocity component v? that is important. The amplitude distribution of this velocity component has the form f v?
1 2 exp v? =vm : pv2m
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It can be demonstrated that at Dt > tp the space± time evolution of initial CF2 concentration distribution (3) is described by the expression n Dt; q n0
R2 R2 v2m Dt
exp
t p 2
!
q2 R2 v2m Dt
t p 2
:
5
The time Dt in expression (5) is measured from the leading edge of the IR pulse. To obtain the value of the normalized signal S Dt; y=S tp ; y 0 measured experimentally, it is necessary to integrate expression (5) for any value of y along the x0 -axis (see Fig. 1). In doing so, one should take account of the ``eective'' FV (the spread function) of the TOF MS. This function is associated with both the purely geometric aperture of the TOF MS and the intensity inhomogeneity of the UV radiation along the x0 -axis. If one changes over from integrating along the x0 -axis to integrating along the x-axis, one then can obtain the following expression for S Dt; y=S0 : S Dt; y=S0 R q exp 2 R2 v2m Dt tp
y2 2 2 R vm Dt
2ka pq 2 p R v2m Dt tp 2 ! Z b x2 exp 2 R2 v2m Dt tp 0 x2 exp dx; a2x
tp
6
where the expression following the ®rst exponent allows for the spread function mentioned above. The form of this function (see Fig. 1) and the parameters b 0:866r (r 0:9 mm is the radius of the geometric aperture of the TOF MS), ax 0:11 cm, and ka 1:04 that characterize it were found in special experiments wherein the UV radiation caustic was moved along the x0 -axis. Thus, we have model relationship (6) with a single ®tting parameter ± the velocity vm .
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The best ®t of the theoretical calculation in accordance with expression (6) in Fig. 6 is provided by vm 650 m/s. The calculated values of S Dt; y=S0 in Figs. 6 and 7 are depicted by smooth curves. A very good agreement between calculations and experiment is everywhere evident. Curve 3 in Fig. 6 demonstrates the role of the spread function. This curve was obtained with the eective FV dierent from that of Fig. 1 and taken to be purely Gaussian with the same value of the parameter ax . The value of the velocity vm found corresponds to the average CF2 radical kinetic energy Etr CF2 3:8 kcal/mol in the centreof-mass frame of reference, and the total kinetic energy of the products (HCl included) is Etr 9 kcal/mol. It should be noted that the energy Etr CF2 3:8 kcal/mol found by us agrees well with the energy Etr CF2 3:5 kcal/mol obtained by the authors of the work reported in Ref. [16], wherein the kinetic energy of the products in experiments on the IR MPD of CF2 HCl was determined by measuring the angular velocity distribution of the CF2 radicals with the aid of a rotatable mass-spectrometer detector. As can be seen from Fig. 6, the escape kinetics of the CF2 radicals remains practically unchanged (within the spread limits of the experimental data points) when the IR radiation ¯uence is increased from U0 6:9 to 10.8 J/cm2 . This is associated with the above-mentioned speci®c feature of the IR MP excitation of small molecules. Because of their rapid dissociation, such molecules cannot be excited too high above the dissociation limit. As shown by many model computations (see, e.g., Ref. [17]), as the radiation ¯uence U is raised, the vibrational distribution being formed ``snuggles'' to some value of Evib Ea , and then there sets in some quasistationary form of this distribution. The molecules in that case dissociate mainly via this upper channel (channels). Such a ``saturation'' of the vibrational excitation level of the molecules results in the corresponding saturation of the amount of energy imparted in the course of dissociation of the molecules to the various degrees of freedom of the products, their translational degrees of freedom included. It is exactly this fact that explains the equality of the vm values for the dierent U0 values in Fig. 6.
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To conclude, let us consider one more aspect of this work. We have already said in the introductory section that an eective beam source of neutral radicals is needed. One of the ways to solve this problem may be the use of the IR MPD of molecules. The results obtained with CF2 HCl show that this approach is promising. Although we have not measured the absolute IR MPD yield in the present work, but judging from the literature data available, with the radiation ¯uence values used by us, namely, U0 10±12 J/cm2 , a good proportion of the CF2 HCl molecules (if not the majority of them) should dissociate. What is important is that such an IR MPD approach can be used to produce beams of accelerated radicals. In this work, we have attained a fairly high velocity of the CF2 radicals (in the centre-of-mass frame of reference), namely, vm 650 m/s, which is more than two times its room-temperature value. So high a velocity of the CF2 radicals is explained by the fact that with the excess of the vibrational energy of the molecule being Evib Ea 6 9 kcal/ mol (around 3 IR quanta), a substantial part of the barrier energy of back reaction (1), DE 6 kcal/ mol, must be transferred to the translational degrees of freedom of the products (Etr 9 kcal/ mol). One can apparently expect that by using precursor molecules with higher threshold energy DE, it will be possible to obtain radicals with still higher velocities.
5. Summary It is found on the basis of our experimental studies that the combined IR UV irradiation of the CF2 HCl molecules by suciently intense laser radiation gives rise to the CF ions. The formation mechanism of these ions is studied, and the conclusion is drawn that they are produced as a result of the IR MPD of CF2 HCl, followed by the fragmentation and subsequent ionization of the resultant CF2 radicals under the eect of UV multiphoton excitation. It is demonstrated that the ®rst step in this UV excitation is resonance ab~ 1 B1 e 1 A1 transition in the sorption in the A X CF2 radical. Such an UV MPI process can be
considered an eective tool for the time-resolved detection of the CF2 radicals. It is used in the present work to study the IR MPD of the CF2 HCl molecules under molecular beam conditions. Gasdynamic cooling is shown to substantially improve the isotopic selectivity of the IR MPD of 13 CF2 HCl. The formation kinetics of the CF2 radicals in the course of the IR MPD of CF2 HCl is studied, and the kinetic energy of the dissociation products measured. The conclusion is drawn that the development of eective beam sources of neutral radicals on the basis of the IR MPD process holds much promise.
Acknowledgements The research described in this paper was made possible in part by grant no. 99-03-33203 of Russian Foundation for Basic Research and by award no. RC1-2206 of the US Civilian Research & Development Foundation for Independent States of the Former Soviet Union (CRDF).
References [1] H. Reisler, C. Wittig, in: K.P. Lawley (Ed.), Photodissociation and Photoionization, Advances in Chemical Physics, vol. 60, Wiley, Chichester, 1985, p. 1. [2] V.S. Letokhov, Laser Photoionization Spectroscopy, Academic Press, Orlando, 1987. [3] M.N.R. Ashfold, S.G. Clement, J.D. Howe, C.M. Western, J. Chem. Soc. Faraday Trans. 89 (1993) 1153. [4] K. Yamasaki, A. Tanaka, A. Watanabe, K. Yokoyama, I. Tokue, J. Phys. Chem. 99 (1995) 15086. [5] J. Stephenson, D. King, M.F. Goodman, J. Stone, J. Chem. Phys. 70 (1979) 4496. [6] A.V. Dem'yanenko, V.N. Lokhman, D.D. Ogurok, E.A. Ryabov, V.S. Letokhov, Chem. Phys. Lett. 320 (2000) 594. [7] M.E. Jacox, J. Phys. Chem. Ref. Data, Monograph N3, 1994. [8] C.W. Mathews, Can. J. Phys. 45 (1967) 2355. [9] V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov, E.A. Ryabov, Multiple Photon Infrared Laser Photophysics and Photochemistry, Harwood Academic Pub., Chur, 1985. [10] E.R. Grant, M.J. Coggiola, Y.T. Lee, P.A. Shultz, Aa.S. Sudbo, Y.R. Shen. Chem. Phys. Lett. 52 (1977) 595. [11] Y.G. McLaughlin, M. Poliako, J.J. Turner, J. Mol. Struct. 82 (1982) 51.
V.N. Lokhman et al. / Chemical Physics 271 (2001) 357±367 [12] H. Okabe, Photochemistry of Small Molecules, Wiley, New York, 1978. [13] D.S. King, J.C. Stephenson, Chem. Phys. Lett. 66 (1979) 33. [14] L.V. Gurvich, G.V. Karachevcev, V.N. Kondrat'ev, Ya.A. Lebedev. V.A. Medvedev, V.K. Potapov, Yu.S. Hodeev. Dissociation energies of chemical bonds. Ionization poten-
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tials, and electron anity. Nauka Pub., Moscow, 1974 (in Russian). [15] J.C. Stephenson, D.S. King, J. Chem. Phys. 69 (1978) 1485. [16] Aa.S. Sudbo, P.A. Schultz, Y.R. Shen, Y.T. Lee, J. Chem. Phys. 69 (1978) 2312. [17] M. Quack, Infrared Phys. 29 (1989) 441.
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Photoionization detection of CF2 radicals resulting from the IR multiple-photon dissociation of CF2HCl molecules in a molecular beam V.N. Lokhman, D.D. Ogurok, E.A. Ryabov * Institute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Moscow region, Russia Received 5 March 2001
Abstract CF ions were detected when irradiating the CF2 radicals resulting from the IR multiple-photon dissociation (MPD) of jet-cooled CF2 HCl molecules with an intense pulsed UV radiation (k 6 244 nm). The experiments performed allowed us to conclude that the CF ions observed were produced in the UV multiphoton excitation of the CF2 radicals via the ~ 1 B1 , followed by their fragmentation and ionization. This eect was used to study the resonant intermediate state A unimolecular decay of CF2 HCl under molecular beam conditions. We measured the kinetics of this process, as well as the translational energies of the CF2 radicals. The isotopic selectivity of the IR MPD of 13 CF2 HCl was found to improve substantially under jet-cooled molecular beam conditions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Multiple-photon ionization (MPI), and its resonance-enhanced version (REMPI) in particular, is being widely used to study highly excited states of molecules and radicals and also various chemical reactions involving polyatomic molecules and radicals (see, e.g., Refs. [1,2], and also the review [3]). Various [n m] REMPI detection schemes were suggested and implemented to date for a large enough number of polyatomic radicals [3]. Speci®cally, the possibility was demonstrated of the REMPI detection of a number of trihalomethyl radicals (see Ref. [3]). These radicals, as well
*
Corresponding author. Tel.: +7-95-334-0231; fax: +7-95334-0886. E-mail address: [email protected] (E.A. Ryabov).
as dihalomethylene radicals, play an important part in the photochemical processes occurring in the environment, including the upper atmosphere and stratosphere. Therefore, the availability of highly sensitive methods for detecting such radicals is important in the study of these processes. In contrast to trihalomethyls, dihalomethylenes are usually detected by the laser-induced ¯uorescence (LIF) technique. In particular, the LIF technique was used for the CF2 radical to study the kinetics of the reaction CF2 O (see Ref. [4] and references cited therein) and also the formation of this radical as a result of the IR multiple-photon dissociation (IR MPD) of CF2 HCl [5]. But the photoionization detection technique can oer a number of obvious new possibilities, as compared to LIF, especially when used in conjunction with massspectrometric analysis, including isotopic analysis possibilities. For this reason, one of the objectives
0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 4 1 7 - 7
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of the present work was to develop a highly sensitive photoionization technique for detecting dihalomethylene radicals and apply it to studies into the kinetics of reactions involving them. The radical to be studied was taken to be CF2 , and the reaction involving it, the formation of 13 CF2 and 12 CF2 as a result of the isotope-selective IR MPD of CF2 HCl, for this molecule was known to dissociate into CF2 and HCl (see, e.g., Ref. [5]). In the above-mentioned REMPI detection of trihalomethyls, use was made of two- or threephoton resonances between visible light quanta and some intermediate state in the particle of interest, a Rydberg state, as a rule (see Ref. [3]), i.e., [2 3 1] REMPI schemes. We propose to detect dihalomethylenes by means of [1 n] schemes of MPI by UV radiation via the ®rst excited singlet state. We have earlier successfully used such a [1 n] photoionization scheme to detect the radical SF5 [6]. For the CF2 radical, the ®rst singlet ~ e abstate (T0 37 226 cm 1 [7]) has its A X sorption band in the region of 250 nm [8]. It was exactly this transition that we used to implement 1 n scheme. This work has one more objective associated with the development of intense sources of radicals. A wide variety of present-day investigations require intense collimated beams of radicals. There are various methods to produce radicals (see, e.g., Ref. [3]). Most frequently used is made of pyrolytic and plasma methods. But they cannot always provide for the necessary concentration of radicals and besides, the precursor molecules in these methods may be subject to an excessive fragmentation. In the case of pulsed beams, the use of photolysis for the production of radicals seems very attractive, both UV and IR radiation being suitable for the purpose. In principle, both these approaches can supplement each other quite well. In the ®rst case, we are dealing with the dissociation of molecules via excited electronic states. The latter circumstance imposes certain requirements upon the type of the laser to be used and can restrict the choice of suitable precursor molecules. In the second case, we have to do with the IR MPD. There are a suciently large number of polyatomic molecules that have their absorption bands in the region of 9±11 lm, so that they can be dis-
sociated by means of a pulsed TEA CO2 laser ± the most common laser covering this wavelength range. The IR MPD process is very ecient: the dissociation yield can be close to unity, and what is more, it proceeds, as a rule, through the rupture of the weakest molecular bond, and so the set of the dissociation products is minimal (see Ref. [9]). Following the ®rst experiments on the IR MPD of SF6 under molecular beam conditions [10], a number of experiments were conducted to study the IR MPD of other jet-cooled molecules. A review of some of these works can be found in Ref. [9]. The analysis of the results obtained in these works shows that the use of the IR MPD process for the production of intense beams of radicals actually appears very promising. Considering everything that has been said above, our experiments on the REMPI detection of the CF2 radical and investigation of the kinetics of the IR MPD of the CF2 HCl molecules were conducted in this work under pulsed molecular beam conditions. The results obtained are presented below.
2. Experimental The experiments were conducted with an experimental set-up similar to the one described in Ref. [6]. Most of the measurements were taken under pulsed molecular beam conditions. The IR multiple-photon excitation and dissociation of the CF2 HCl molecules were carried out using a line-tunable pulsed TEA CO2 laser. To photoionize the IR MPD products, use was made of a pulsed UV radiation. The ions thus produced were detected by means of a time-of-¯ight mass spectrometer (TOF MS). The irradiation geometry is illustrated in Fig. 1. The molecular and laser beams lay in one and the same plane and intersected on the TOF MS axis (y-axis) that was perpendicular to this plane. The supersonic beam of gas-dynamically-cooled CF2 HCl molecules (a General Valve nozzle, 0.8-mm-diameter ori®ce, 200-ls pulse, fwhm) was directed into the ionization chamber through a 0.8-mm-diameter skimmer. The typical stagnation pressure was 2 atm. The molecular density distribution along the x0 - and
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Fig. 1. Illustrating the IR UV irradiation of the CF2 HCl molecules in a beam. At the top and side are the distributions of the concentration of the molecules (M) and IR radiation intensity along the x0 - and y-axis, respectively. Shown at the top is also the form of the eective FV of the TOF MS (for details, see text).
y-axis in the beam is presented in Fig. 1. It is of a trapezoidal shape with a width (fwhm) of 5 mm along the y-axis, the width of its ¯at portion being equal to 3 mm. The IR-laser radiation (pulse duration 130 ns, fwhm) was directed opposite to the molecular beam and at a small angle to its z-axis. It was focused into the vacuum chamber by means of a telescope system. The IR radiation intensity distribution in the focal plane was Gaussian to a good approximation, its diameter (at the e 1 level) being approximately equal to 0.8 mm. The length of the caustic was around 2 cm, and the laser ¯uence U0 on the beam axis could reach as high a value as 12 J/cm2 . The frequency-doubled output from a XeCllaser-pumped dye laser was used as a UV radiation source. The laser used Coumarin-47 and Coumarin-102 dyes and a BBO-crystal frequency doubler to have its UV output tunable in the range 225± 260 nm. The UV pulses (energy 120±150 lJ, duration 10 ns (fwhm)) were focused with a lens
(f 12 cm) into the vacuum chamber where they intersected the molecular beam at an angle of 60° (x0 -axis in Fig. 1). The cross-sectional area of the UV beam in the focal region amounted to some 10 5 cm2 . As noted earlier, the ions produced were detected and identi®ed by means of a TOF MS. The top curve of Fig. 1 represents the eective ®eld of vision (FV) of the TOF MS for the ionizing UV radiation geometry used. It can be seen from the distribution curves presented in Fig. 1 that the molecular beam was irradiated in its homogeneous region. The inhomogeneity of the IR radiation and the shape of the FV of the TOF MS were taken into consideration when processing the results of measurements (see elsewhere). The pulsed nozzle and the CO2 laser were operated in step so as to make the IR MPD of the molecules occur at the peak of the molecular pulse (DT ' 0:78 ms). By varying the delay time Dt between the IR and UV pulses, we could study the kinetics of the formation and subsequent evolution
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of the IR MPD products (see elsewhere). In some cases, measurements were taken with long delay times (DT ' 4:6 ms) between the molecular and CO2 laser pulses, i.e., well after the former came to an end. In this case, subject to IR UV irradiation were the particles that had repeatedly suered scattering by the chamber walls and thus reached a temperature approximately equal to the room value. The signals from the TOF MS as well as from IR and UV pulse energy detectors were monitored by a digital oscilloscope and then fed to a computer to be accumulated and processed. In our experiments, we used 99.8% pure CF2 HCl with natural concentrations of the isotopes 13 C and 12 C (1.1% and 98.9%, respectively).
3. Measurement results The IR irradiation of the CF2 HCl was eected in the 9.3-lm CO2 laser region where the symmetric m3 (1100 cm 1 ) and antisymmetric m8 (1118 cm 1 ) modes of the C±F bonds of this molecule are located [11]. The isotope shift with respect to the m3 mode comes to 24 cm 1 m3 13 CF2 HCl 1076 cm 1 , and this allowed us to tune in to both 12 Cand 13 C-containing molecules. We found that when the energy ¯uence of the CO2 laser was high enough, U0 > 3:0 J/cm2 , and the UV radiation wavelength k was shorter than 244 nm, the combined IR UV irradiation of the molecules gave rise to the CF ions, no other ionic products being detected. Fig. 2 presents the mass spectra obtained in the IR UV irradiation of CF2 HCl at the 9R 8 CO2 -laser line (1070.46 cm 1 ). The spectra were obtained with the molecular gas at room temperature (Fig. 2a) and with the gas being gas-dynamically cooled (Fig. 2b). The IR radiation frequency selected fell within the spectral pro®le of the IR MPD of 13 CF2 HCl, and so there should have predominantly occurred the dissociation of this isotopic molecule. This fully corresponds to the data of Fig. 2. Speci®cally, the 13 CF and 12 F peak intensities in Fig. 2b approximately coincide, whereas the 13 C0 =12 C0 ratio in the parent molecule amounts to 0.011. The quantitative measure of
Fig. 2. Mass spectra of the ions produced in the course of the IR UV irradiation of the CF2 HCl molecules: (a) at room temperature; (b) under gas-dynamic cooling conditions. U0 12 J/cm2 , 9R 8 CO2 laser line.
the selectivity of the process is the parameter a 13=12: a 13=12
13 CF t 13 C0 : : 12 CF t 12 C0
Under irradiation conditions of Fig. 2, the selectivity a 13=12 18 for the molecular gas at room temperature. As one would expect, in the case of gas-dynamic cooling of the molecules, the selectivity of their IR MPD process increases substantially, up to a 13=12 45:5. To enhance the CF -ion signal, all the subsequent measurements were taken at the 9R 30 CO2 -laser line (1084.64 cm 1 ) where it was 12 CF2 HCl that predominantly suered dissociation. Fig. 3 shows the dependence of the CF -ion signal on the IR radiation ¯uence. The form of this relationship is quite typical of the IR MPD process (see, e.g., Ref. [9]). At the beginning there is a region where the dependence is very strong. In our case, it is the region of U0 3 J/cm2 , which can be
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361
Fig. 4. CF -ion signal as a function of the UV radiation wavelength. The vertical lines indicate the position of the ~ 1 B1 e 1 A1 transitions in CF2 (according to t02 t002 0 A X the data presented in Ref. [8]). Shown in the inset is the smallscale structure in the t02 12 t002 0 transition region. U0 6:6 J/cm2 ; EUV 90 lJ. Fig. 3. CF -ion signal as a function of the IR radiation ¯uence. kUV 230:24 nm, EUV 70 lJ, Dt 0:51 ls.
interpreted as some threshold. Next follows a region where the dependence is weaker and can be approximated in some region of variation of the ¯uence U by a power function. For the data of Fig. 3, the power of this function is n 1:85. We also studied the dependence of the CF -ion signal on the wavelength and intensity of the UV radiation. Fig. 4 presents the spectral dependence of the CF -ion yield. As noted earlier, under our experimental conditions the CF ions could reliably be detected at an UV radiation wavelength of kUV 6 244 nm. As can be seen from Fig. 4, the spectral dependence obtained features a distinct structure with a period of Dm ' 500 cm 1 . In the course of our measurements, we found that the spectrum includes, in addition to this large-scale structure, also some small-scale one. A typical example of such a structure is presented in the inset in Fig. 4. The origin of these structures will be discussed in Section 4. Fig. 5 shows the dependence of the CF -ion signal on the UV radiation
Fig. 5. CF -ion signal as a function of the UV radiation energy. () room temperature; ( ) jet-cooled molecules. U0 8:5 J/cm2 , Dt 0:51 ls.
energy (intensity). The measurements were taken in the region of the maximum of the UV spectrum measured (see Fig. 4). The dependence of Fig. 5 was plotted for both the molecular beam and scattered molecular gas. In the latter case, the CF -ion signal is approximately 3.5 times as high
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as in the case of molecular beam, but the data presented in Fig. 5 were appropriately normalized. It can be seen that at EUV P 50 lJ, which corresponds to UUV ' 5 J/cm2 in the caustic of the UV beam, the yield of the CF ions rises approximately linearly with increasing UV radiation intensity. In the region of lower EUV values, this dependence is apparently stronger, this being testi®ed to by the experimental data points in hand. Unfortunately, we failed to measure this dependence more accurately, our measurement sensitivity being limited. As already noted, by varying the delay time Dt between the IR and UV pulses and changing the location of the probe UV beam, one can study the kinetics of the formation and subsequent space± time evolution of the precursor radicals of the CF ions observed. We believe (see Section 4) that the precursor of the CF ion is the CF2 radical formed as a result of the IR MPD of the CF2 HCl molecule. The pertinent relationships between the CF signal and the delay time Dt are presented in Fig. 6 for two IR radiation ¯uence values, namely, U0 6:9 and 10.8 J/cm2 . The measurements were taken with intersecting IR and UV beams, i.e., with zero displacement of the UV beam along the y-axis of Fig. 1. The leading edge of the curve obtained is associated with the formation of the CF2 radicals, and the subsequent reduction of the signal is
Fig. 7. Evolution of the spatial distribution of the CF2 radicals. Normalized CF -ion signal S Dt; y 0=S0 as a function of the distance y to the IR beam axis for various delay times Dt. The smooth curves ± model computation for vm 650 m/s. U0 10:8 J/cm2 , kUV 234:31 nm, EUV 90 lJ.
caused by their escape from the dissociation region. The latter conclusion is supported by the data of Fig. 7 which re¯ects the spatial evolution of the CF2 radicals. Presented in this ®gure are the relationships between the CF signal and the displacement of the probe UV beam along the y-axis (see Fig. 1) at various instants of time Dt. While at Dt 0:44 ls (peak in the curve of Fig. 6) the spatial distribution of the CF2 radicals does not fall beyond the limits of the CO2 laser beam, it clearly spreads wider as Dt is increased. The quantitative interpretation of these changes is presented below. 4. Discussion
Fig. 6. Normalized CF -ion signal S Dt; y 0=S0 as a function of the delay time Dt between the IR and UV pulses. (1) Shape of the CO2 laser pulse; (2) model computation; (3) model computation for the modi®ed spread function (see text). kUV 234:31 nm; EUV 90 lJ.
In our view, the results obtained allow us to draw the unambiguous conclusion that the CF ions observed are formed as a result of a series of consecutive processes, including the IR MPD of the CF2 HCl molecules and the subsequent fragmentation and ionization of the resultant CF2 radicals under the eect of the UV radiation they absorb in a multiple-photon fashion. In other words, there take place the following processes: CF2 HCl nhmIR ! CF2 HCl;
1
V.N. Lokhman et al. / Chemical Physics 271 (2001) 357±367
CF2 mhmUV ! CF F e:
2
Indeed, the CF2 HCl molecules themselves make no contribution to the formation of the CF ions. The UV absorption band of these molecules starts in the region of 210 nm [12], which is much shorter than the UV radiation wavelength used by us. Besides, special experiments have demonstrated that the CF ions could not be observed in the absence of IR radiation. Highly vibrationally excited CF2 HCl molecules could, in principle, contribute to the formation of these ions. The IR MP excitation of molecules has been known (see, e.g., Ref. [9]) to give rise to a suciently wide vibrational distribution, up to the dissociation limit and higher. But in our case such highly excited CF2 HCl do not contribute to the formation of CF . If this had taken place, in the plot of the CF signal as a function of Dt (see Fig. 6) there would have been observed a substantial ``nonvanishing'' (in time) signal component associated with the absence of the escape of the excited molecules from the IR excitation region. Actually, as can be seen from Fig. 6, there takes place a rather rapid reduction of the signal to very low values, and the kinetics of this process is fully explained by the escape of the CF2 radicals (see later). Thus, the only precursors of the CF ions are the CF2 ions formed as a result of reaction (1). This conclusion, too, fully agrees with the character of the relationship between the CF -ion signal and the IR radiation ¯uence. As noted earlier, there occurs a yield-¯uence relationship typical of the IR MPD process. The CF signal arises only when the IR MPD threshold is surpassed. According to the data presented in Ref. [13], the magnitude of this threshold for CF2 HCl is around 1 J/cm2 . In our experiments, the CF signal arose at U0 P 3 J/cm2 . For a Gaussian beam, this corresponds to an average ¯uence of U 1:5 J/cm2 (it is exactly this averaged parameter that is usually cited in the literature), which, allowing for some dierences between the experimental conditions, agrees quite well with the above literature value. Note also that the data on the isotopic selectivity of the process (Fig. 2) also agree with the conclusion that the ®rst step in the formation of the CF ions is the IR MPD of the CF2 HCl molecules, i.e., reaction (1).
363
The formation of the CF from the CF2 radicals (reaction (2)) occurs as a result of the multiplephoton absorption of UV radiation. One can estimate the energy required for this process, assuming that it is the dissociation of the CF2 radicals that is the ®rst to take place, the CF radicals thus produced being then subject to ionization. The quantity D0 CF±F 41 273 cm 1 [14], and the ionization potential of CF is I0 9:11 eV. Thus, to produce a CF ion, one has to expend no less than 11 4752 cm 1 of energy, which is slightly less than the energy of three quanta of the UV radiation used by us. In actual fact, the formation of a CF ion apparently requires more than three UV quanta. We believe that the ®rst step in such a multiphoton (multistep) process is the resonance ~ 1 B1 e 1 A1 in CF2 . The one-photon transition A X spectroscopy of this transition has been well studied [8]. Its absorption spectrum lies mainly in the range 230±280 nm and features a well-de®ned t02 t002 vibrational structure associated with the m2 bending vibration whose frequency in the upper electronic state is m02 496 cm 1 . The positions of the maxima of the pertinent bands, t02 t002 0, are shown in Fig. 4. Obviously the position of these transitions matches well enough the structure observed by us. At the same time, the transitions of Fig. 4 are perceptibly wider than their counterparts measured in Ref. [8]. Our analysis indicates that this is due to the higher vibrational and rotational temperatures of the CF2 radicals in our experiments. According to the data presented in Ref. [15], the IR MPD of the CF2 HCl molecules gives rise to suciently ``hot'' CF2 radicals with vibrational and rotational temperatures of 1160 and 2000 K, respectively. It should also be noted that on the whole the shape (envelope) of the onephoton absorption spectra presented in Ref. [8] and in Fig. 4 dier noticeably. The former has a maximum in the region of 245±248 nm, whereas the one measured by us has already terminated in this region. In our view, this agrees with the multiphoton/multistep character of reaction (2): it is exactly the subsequent transitions that determine the long-wavelength boundary of the spectrum in Fig. 4. As for the small-scale structure in aour spectrum, it is apparently associated with the rotational structure of the transitions, which
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clearly manifest itself in the one-photon absorption spectrum presented in Ref. [8]. Considering the multiphoton character of the CF ion formation process, it is dicult to interpret this structure more accurately. It is our belief that the multiphoton character of the CF -ion formation process does not contradict the character of relationship measured by us between the CF -ion signal and the UV radiation intensity (see Fig. 5). In the region of low UV radiation intensities, the nonlinear character of this relationship is clearly manifest. The subsequent passage to an almost linear dependence is associated with the saturation of the quantum transitions. Our estimates show that at least the ®rst, resonance transition is always strongly saturated, no matter what the value EUV from the range of Fig. 5. The data available on the structure of the highlying levels in CF2 are inadequate to unambiguously describe the subsequent transitions (those ~ 1 B1 e 1 A1 ) occurring following the ®rst one, A X under the eect of UV radiation. One of the possible versions is as follows. The CF2 radical has a e with an energy of T0 72 740 cm 1 [7], to state B ~ 1 B1 . If the which it can pass from its excited state A e state B is a dissociating one, the CF ion can then be formed by way of the two-photon ionization of the resultant CF radical. Other versions are possible as well, but in any case we think that most likely it is the dissociation of CF2 that is the ®rst to occur, with the ionization of CF close behind, no CF 2 ion being observed in our experiments. Let us now consider the space±time evolution of the CF signal illustrated in Figs. 6 and 7. As follows from what has been said above, it is associated with the space±time evolution of the CF2 radicals formed in the course of reaction (1). So, by measuring the signal produced by the CF ions resulting from the UV MPI of CF2 , one can study the unimolecular decay kinetics of CF2 HCl under the eect of IR radiation. The dissociation of CF2 HCl is a three-centre HCl elimination reaction, with the back reaction having a barrier. According to Ref. [16], the activation energy of direct reaction (1) is Ea 54 kcal/ mol, the back reaction barrier being DE 6 kcal/ mol. As with many relatively small molecules, the
dissociation lifetime s of CF2 HCl shortens rapidly as the excess of its vibrational energy Evib over the activation energy Ea grows higher. For example, according to the RRKM-theory calculations made in Ref. [16], when Evib exceeds Ea by 1 IR quantum, the lifetime s is around 50 ns, and it shortens down to 5 ns when the excess of Evib over Ea equals 3 IR quanta. Because of their high-dissociation rate, such molecules cannot be overexcited too much above the dissociation limit. Simple estimates show that under our experimental conditions CF2 HCl molecules can hardly be excited to a level lying above Ea by more than 3 IR quanta. Therefore, considering what has been said above, it can be taken that in the experimental conditions of Figs. 6 and 7 all the molecules had already dissociated by the moment the IR pulse ended. The leading edge of the kinetic curve in Fig. 6 is mainly determined by the duration of the IR pulse, and the subsequent reduction of the signal is caused by the escape of the CF2 radicals from the IR excitation region. By modelling this process and comparing between the theoretical and experimental results obtained, we can obtain some parameters of reaction (1). In our model, it was assumed that by the instant tp when the IR pulse came to an end all the CF2 HCl molecules had already dissociated. The initial CF2 concentration distribution measured (in the same way as the data of Fig. 7) at this moment of time is described well enough by the Gaussian function n tp ; q n0 exp q2 =R2 ;
3
where q2 x2 y 2 (see Fig. 1), and R 0:347 mm. The evolution of the spatial distribution of the concentration of the CF2 radicals was assumed to take place as a result of their free motion in accordance with their velocity distribution. We supposed that there was a Maxwell velocity distribution with a most probable velocity of vm . In our experimental geometry, it is only the transverse (normal to the z-axis) velocity component v? that is important. The amplitude distribution of this velocity component has the form f v?
1 2 exp v? =vm : pv2m
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V.N. Lokhman et al. / Chemical Physics 271 (2001) 357±367
It can be demonstrated that at Dt > tp the space± time evolution of initial CF2 concentration distribution (3) is described by the expression n Dt; q n0
R2 R2 v2m Dt
exp
t p 2
!
q2 R2 v2m Dt
t p 2
:
5
The time Dt in expression (5) is measured from the leading edge of the IR pulse. To obtain the value of the normalized signal S Dt; y=S tp ; y 0 measured experimentally, it is necessary to integrate expression (5) for any value of y along the x0 -axis (see Fig. 1). In doing so, one should take account of the ``eective'' FV (the spread function) of the TOF MS. This function is associated with both the purely geometric aperture of the TOF MS and the intensity inhomogeneity of the UV radiation along the x0 -axis. If one changes over from integrating along the x0 -axis to integrating along the x-axis, one then can obtain the following expression for S Dt; y=S0 : S Dt; y=S0 R q exp 2 R2 v2m Dt tp
y2 2 2 R vm Dt
2ka pq 2 p R v2m Dt tp 2 ! Z b x2 exp 2 R2 v2m Dt tp 0 x2 exp dx; a2x
tp
6
where the expression following the ®rst exponent allows for the spread function mentioned above. The form of this function (see Fig. 1) and the parameters b 0:866r (r 0:9 mm is the radius of the geometric aperture of the TOF MS), ax 0:11 cm, and ka 1:04 that characterize it were found in special experiments wherein the UV radiation caustic was moved along the x0 -axis. Thus, we have model relationship (6) with a single ®tting parameter ± the velocity vm .
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The best ®t of the theoretical calculation in accordance with expression (6) in Fig. 6 is provided by vm 650 m/s. The calculated values of S Dt; y=S0 in Figs. 6 and 7 are depicted by smooth curves. A very good agreement between calculations and experiment is everywhere evident. Curve 3 in Fig. 6 demonstrates the role of the spread function. This curve was obtained with the eective FV dierent from that of Fig. 1 and taken to be purely Gaussian with the same value of the parameter ax . The value of the velocity vm found corresponds to the average CF2 radical kinetic energy Etr CF2 3:8 kcal/mol in the centreof-mass frame of reference, and the total kinetic energy of the products (HCl included) is Etr 9 kcal/mol. It should be noted that the energy Etr CF2 3:8 kcal/mol found by us agrees well with the energy Etr CF2 3:5 kcal/mol obtained by the authors of the work reported in Ref. [16], wherein the kinetic energy of the products in experiments on the IR MPD of CF2 HCl was determined by measuring the angular velocity distribution of the CF2 radicals with the aid of a rotatable mass-spectrometer detector. As can be seen from Fig. 6, the escape kinetics of the CF2 radicals remains practically unchanged (within the spread limits of the experimental data points) when the IR radiation ¯uence is increased from U0 6:9 to 10.8 J/cm2 . This is associated with the above-mentioned speci®c feature of the IR MP excitation of small molecules. Because of their rapid dissociation, such molecules cannot be excited too high above the dissociation limit. As shown by many model computations (see, e.g., Ref. [17]), as the radiation ¯uence U is raised, the vibrational distribution being formed ``snuggles'' to some value of Evib Ea , and then there sets in some quasistationary form of this distribution. The molecules in that case dissociate mainly via this upper channel (channels). Such a ``saturation'' of the vibrational excitation level of the molecules results in the corresponding saturation of the amount of energy imparted in the course of dissociation of the molecules to the various degrees of freedom of the products, their translational degrees of freedom included. It is exactly this fact that explains the equality of the vm values for the dierent U0 values in Fig. 6.
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To conclude, let us consider one more aspect of this work. We have already said in the introductory section that an eective beam source of neutral radicals is needed. One of the ways to solve this problem may be the use of the IR MPD of molecules. The results obtained with CF2 HCl show that this approach is promising. Although we have not measured the absolute IR MPD yield in the present work, but judging from the literature data available, with the radiation ¯uence values used by us, namely, U0 10±12 J/cm2 , a good proportion of the CF2 HCl molecules (if not the majority of them) should dissociate. What is important is that such an IR MPD approach can be used to produce beams of accelerated radicals. In this work, we have attained a fairly high velocity of the CF2 radicals (in the centre-of-mass frame of reference), namely, vm 650 m/s, which is more than two times its room-temperature value. So high a velocity of the CF2 radicals is explained by the fact that with the excess of the vibrational energy of the molecule being Evib Ea 6 9 kcal/ mol (around 3 IR quanta), a substantial part of the barrier energy of back reaction (1), DE 6 kcal/ mol, must be transferred to the translational degrees of freedom of the products (Etr 9 kcal/ mol). One can apparently expect that by using precursor molecules with higher threshold energy DE, it will be possible to obtain radicals with still higher velocities.
5. Summary It is found on the basis of our experimental studies that the combined IR UV irradiation of the CF2 HCl molecules by suciently intense laser radiation gives rise to the CF ions. The formation mechanism of these ions is studied, and the conclusion is drawn that they are produced as a result of the IR MPD of CF2 HCl, followed by the fragmentation and subsequent ionization of the resultant CF2 radicals under the eect of UV multiphoton excitation. It is demonstrated that the ®rst step in this UV excitation is resonance ab~ 1 B1 e 1 A1 transition in the sorption in the A X CF2 radical. Such an UV MPI process can be
considered an eective tool for the time-resolved detection of the CF2 radicals. It is used in the present work to study the IR MPD of the CF2 HCl molecules under molecular beam conditions. Gasdynamic cooling is shown to substantially improve the isotopic selectivity of the IR MPD of 13 CF2 HCl. The formation kinetics of the CF2 radicals in the course of the IR MPD of CF2 HCl is studied, and the kinetic energy of the dissociation products measured. The conclusion is drawn that the development of eective beam sources of neutral radicals on the basis of the IR MPD process holds much promise.
Acknowledgements The research described in this paper was made possible in part by grant no. 99-03-33203 of Russian Foundation for Basic Research and by award no. RC1-2206 of the US Civilian Research & Development Foundation for Independent States of the Former Soviet Union (CRDF).
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