- Email: [email protected]
Vacuum-UV fluorescence spectroscopy of polyatomic molecules
Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 101–107
Vacuum-UV fluorescence spectroscopy of polyatomic molecules ¨ b , H.W. Jochims b R.P. Tuckett a , *, H. Biehl a , K.J. Boyle a , D.P. Seccombe a , H. Baumgartel a
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2 TT, UK b ¨ f ur ¨ Physikalisch Chemie, Freie Universitat ¨ Berlin, D-14195 Berlin, Germany Institut Received 4 August 1998
Abstract Experiments are described in which UV and visible fluorescence is observed following vacuum-UV photoexcitation of a range of polyatomic molecules in the gas phase. Tunable radiation in the energy range 8–25 eV from synchrotron sources at Daresbury, UK and BESSY 1, Germany is used as the photoexcitation source. Non-dispersed fluorescence excitation spectra and dispersed fluorescence spectra are recorded at Daresbury and BESSY 1, respectively. The experiments are sensitive to Rydberg states of molecules that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresces. Using single-bunch mode of BESSY 1, lifetimes of the emitters in the range ca. 3–100 ns are measured. Examples are taken from work either published or in press in SiF 4 and GeF 4 , PF 3 , MCl 4 (M5C, Si, Ge), and CXCl 3 (X5H, F, Br). 1999 Elsevier Science B.V. All rights reserved.
ABn 1 hv1 → (ABn )* → (ABn21 )* 1 B
1. Introduction In this second paper we describe experiments in which UV and visible fluorescence is observed following vacuum-UV excitation of a range of polyatomic molecules. The two groups comprising the authors have collaborated to study the photodissociation dynamics of both the Rydberg states of polyatomic molecules with four to six atoms and the valence states of their parent molecular ion. Specifically, our experiments are sensitive to those Rydberg states that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresce. If ABn represents a general polyatomic molecule, these two processes can be represented as:
*Corresponding author. Tel.: 144-121-414-425; fax: 144-121414-4403. E-mail address: [email protected] (R.P. Tuckett)
or (ABn22 )* 1 2B, (ABn21 )* or (ABn22 )* → ABn21 or ABn22 1 hn2 (1) and ABn 1 hv1 → (AB n1 )* 1 e 2 , 1 (AB 1 n )* → AB n 1 hv 2
(2)
hn1 and hn2 represent photons in the vacuum ultraviolet (VUV, 50, l1 ,150 nm or 8,E1 ,25 eV) and ultraviolet / visible (200, l2 ,700 nm), respectively. Processes 1 and 2 lead to resonant and non-resonant peaks, respectively, in the VUV fluorescence excitation spectrum of ABS. Process 1 is termed resonant, because the VUV photon must populate resonantly the Rydberg state of ABn . Process 2 is termed nonresonant because, as in photoelectron spectroscopy, the signal is still observed for photon energies well
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00419-8
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in excess of the threshold to produce (AB 1 n )* because the electron can remove the excess energy. These two processes lead to different peak shapes in the fluorescence excitation spectrum [1]. Using synchrotron radiation, it has been possible to populate, state-selectively and often at vibrational resolution, Rydberg states and valence molecular ion states of a range of polyatomic molecules in the energy range ca. 8–25 eV for such studies. In some circumstances we have been able to comment on the detailed photodissociation dynamics of Rydberg states. This paper summarises the important results for SiF 4 [2] and GeF 4 [3], PF 3 [4], MCl 4 (M5C, Si, Ge) [5], and CXCl 3 (X5H, F, Br) [6]. From an applied scientific viewpoint, many of these molecules are used in the plasma etching of integrated circuits, and our results should be of use to scientists studying such processes. In RF discharges, neutral free radicals and molecular ions are created in an ill-defined way by low-energy electrons whose energies span the wide range ca. 3–50 eV. In this project, we use the more controllable method of photoexcitation with photons spanning the same energy range as the plasma electrons. Our results could have diagnostic applications in the monitoring of species present in plasmas.
Fig. 1. Schematic of the non-dispersed fluorescence excitation apparatus used at Daresbury.
of the electronics limits the lifetimes that can be measured to ca. 3–100 ns. Three characteristics of the Daresbury experiment are important. First, flux normalisation to the VUV
2. Experimental Experiments are performed at synchrotron sources at Daresbury, UK and BESSY 1, Germany. Full details are given elsewhere [2,4]. Non-dispersed, fluorescence excitation spectra are recorded at Daresbury (Fig. 1) using either a 1 m Seya-Namioka (range 8–35 eV, best resolution 0.05 nm) or a 5 m normal-incidence McPherson monochromator (range 8–30 eV, best resolution 0.01 nm) attached to the 2 GeV electron storage ring as the primary source of tunable VUV radiation. Dispersed fluorescence and VUV action spectra are recorded at BESSY 1 (Fig. 2), using a 1.5 m normal-incidence monochromator (range 7–25 eV, optimum resolution 0.03 nm) attached to the 800 MeV electron storage ring to monochromatise the radiation. Using the singlebunch mode of BESSY 1 (20 ps pulses every 208 ns), we measure the lifetimes of the emitting states, but the timing profile of the source and response time
Fig. 2. Schematic of the dispersed fluorescence apparatus used at BESSY 1.
R.P. Tuckett et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 101 – 107
photon beam is performed in situ using a sodium salicylate window and a photomultiplier tube. Second, the use of optical filters can isolate the range of wavelengths over which undispersed fluorescence is detected by the photon-counting EMI 9813 QB photomultiplier tube (range 190–650 nm), in effect affording a low-resolution action spectrum. Third, the experiment gives information on the spectroscopy of the Rydberg states of the polyatomic molecule, valence states of the parent ion that decay radiatively, and fluorescence excitation functions. A limited amount of data only is obtained about the spectroscopy of the emitting species. There are also three important characteristics of the BESSY 1 experiment. First, it is difficult to flux-normalise fluorescence excitation spectra in situ to the VUV photon beam. Second, a 0.2 m secondary monochromator (Jobin Yvon H2OVIS) with no entrance slit and an exit slit of 1–2 mm, giving a resolution of ca. 4–8 nm, disperses the induced fluorescence. Experiments can disperse the fluorescence after excitation at a fixed (primary) VUV energy, giving low-resolution electronic spectra of the emitting species. Action spectra, where the VUV energy is scanned for detection at a known secondary wavelength, can also be obtained. Third, the ability to disperse the induced fluorescence, determine the wavelength of the emission and hence define the emitter, makes this an ideal apparatus for measuring
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lifetimes of fluorescing states of neutral free radicals and molecular ions. This is particularly true when more than one emitter is produced at any VUV photon energy.
3. SiF 4 and GeF 4 The plasma dry etching of silicon wafers by fluorine-containing gases (e.g. CF 4 , C 2 F 6 ) has applications to the fabrication of microelectronic devices. The role of the SiF 2 and SiF 3 ? radicals in the etching process is known to be important [7], hence a knowledge of the electronic spectroscopy of these species is needed. To date, the electronic spectroscopy of SiF 3 ? is very poorly understood, and there are major gaps in our knowledge of the excited states of SiF 2 . Both resonant and non-resonant peaks are observed in the VUV fluorescence excitation spectrum of SiF 4 between 10 and 25 eV [2]. Transitions to low-lying Rydberg states in this range that photodissociate to a fluorescing state of a fragment radical have all been assigned (Table 1). Fluorescence has been observed as valence transitions in SiF 3 ?, in SiF 2 , and, especially strongly, in the parent ion SiF 41 . Fluorescence from excited states of the SiF radical is thermochemically forbidden, and has not been observed. From the dispersed spectra obtained at
Table 1 Peak positions and assignments from fluorescence excitation spectroscopy of the Rydberg and ionic states of SiF 4 in the range 10–25 eV, and assignments of the fluorescing fragments and ions E / eV a 13.0 13.9 14.8 15.95 18.1 19.45 .21.5 c
Assignment 21
(IE 2 E) / eV
(1t 1 ) 3p 3.5 or (3t 2 )21 3s 4.4 (3t 2 )21 3p 3.5 (1t 1 )21 3d 1.7 or (1e)21 3p 3.2 (3t 2 )21 3d 1.45 or (3t 2 )21 4p 1.45 (2t 2 )21 4p 1.36 (2a 1 )21 4s 2.1 2 ˜ 2 (2a 1 )21 →SiF 1 4 D A 1 1e
(n2d )
db
Emission range / nm
Emitter
1.97 1.76 1.97 2.83 2.06 3.06 3.06 3.16 2.54
1.03 1.24 1.03 0.17 0.94 20.06 0.94 0.84 1.46
ca. 380– 800
SiF 3 ?*
ca. 380–800 ca. 380– 800
SiF 3 ?* SiF 3 ?*
ca. 360–440
SiF 2 a˜ 3 B 1
ca. 220–280 ca. 220–280 ca. 280–350
˜ 1B 1 SiF 2 A ˜ 1B 1 SiF 2 A 1 ˜ 2 SiF 4 D A 1
˜ 2 A 1 state of SiF 1 The effects of second-order radiation from the primary monochromator producing emission from the D 4 at excitation energies less than 21.5 eV [2] are ignored in this table. b Quantum defect, d, defined by the equation E5IE2[R H /(n2d )2 ], where R H is the Rydberg constant and n is the principal quantum number of the Rydberg orbital. Calculated using the appropriate vertical ionisation energies (IE) for SiF 4 from threshold photoelectron spectroscopy [9]. c Threshold for fluorescence, not peak position. a
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BESSY 1, four different decay channels are observed:
1. SiF 3 ? fluorescence in the range 380–800 nm for VUV photon energies around 13.0 eV, ˜ 1 A phosphorescence from 360–440 2. SiF 2 a˜ 3 B 1 –X 1 nm for VUV energies in the range 15.2–18.0 eV, ˜ 1 B –X ˜ 1 A fluorescence from 210–270 3. SiF 2 A 1 1 nm for VUV energies in the range 17.0–20.0 eV, ˜ 2 A state of SiF 1 4. intense emission from the D 1 4 predominantly in the range 280–350 nm for VUV energies greater than the adiabatic ionisation energy of this state, 21.5 eV. The lifetimes of the unassigned emitting state in ˜ 1 B state of SiF and the D ˜ 2 A state of SiF 3 ?, the A 1 2 1 SiF 41 have been measured to be 3.9, 11.2 and 9.16 ns, respectively (Table 2) [2]. The SiF 3 ? result is new, the symmetry of the emitting state is unknown, and it is not even clear whether emission is occurring to the ground or an excited state of the radical. This work has highlighted the urgent need for high quality ab initio calculations on the energies and geometries
of excited states of SiF 3 ?. There is one surprising 3 lifetime result. The decay from the a˜ B 1 state of SiF 2 has a fast component of 3.3 ns (Table 2) [3]. We conclude that the radiative lifetime of this state is either as low as 3.3 ns or too high (t .ca. 500 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for spin-forbidden phosphor˜ 1 A ground state, the 3.3 ns escence to the X 1 component could be the lifetime of inter-system crossing from higher vibrational levels of the a˜ 3 B 1 state of SiF 2 into its ground state [2,8]. With GeF 4 , only three decay channels are ob3 ˜ 1 A phosphorescence, served [3]: GeF 2 a˜ B 1 –X 1 1 1 ˜ ˜ GeF 2 A B 1 –X A 1 fluorescence, and emission from ˜ 2 A state of GeF 1 . There is no analagous the D 1 4 emission in the GeF 3 ? radical. With SiF 4 there is no evidence whether SiF *2 forms with F 2 or 2F as the other product(s) of SiF *4 photodissociation. With GeF 4 , however, there is strong evidence from ther˜ 1 B states of mochemistry that both the a˜ 3 B 1 and A 1 GeF 2 form via a one-step mechanism with F 2 , and ˜ 1B not 2F, as the other product. Lifetimes of the A 1 ˜ 2 A state of GeF 1 are state of GeF 2 and the D 1 4 measured to be 9.3 and 5.02 ns, respectively (Table
Table 2 Lifetimes of emission bands observed from VUV excitation of SiF 4 (top), GeF 4 (middle) and PF 3 (bottom) in the range 10–25 eV E1 / eV
l2 / nm (68 nm)
t / ns
Reduced x 2
Emitter
13.0 15.9 18.1 21.8
480 400 225 310
3.960.7 3.360.4 11.261.5 9.1660.02
1.71 1.50 2.01 1.33
SiF 3 ?* SiF 2 a˜ 3 B 1 a ˜ 1B 1 SiF 2 A 1 ˜ 2 SiF 4 D A 1 b
14.8 15.9 21.4
340 235 255
Flat decay c 9.360.1 5.0260.01
2.67 1.07
GeF 2 a˜ 3 B 1 ˜ 1B 1 GeF 2 A 2 ˜ GeF 1 D A1 d 4
9.8 11.0 14.4 14.4 16.1
450 300 222 325 322
Flat decay Flat decay 14.760.1 7.960.1 Flat decay
1.55 1.70
˜ 2 A 1 ( 2 P)e PF 2 ? A ˜ 2B 2 PF 2 ? B ˜ 2A 1 PF 2 ? C ˜ PF 2 ? E 2 B 1 ( 2 P)e PF A 3 P
a Almost certainly, this value is the lifetime for inter-system crossing of vibrational levels of SiF 2 a˜ 3 B 1 to high vibrational levels of the electronic ground state, not the radiative lifetime of the a˜ 3 B 1 state. b ˜ 2 A 1 –A ˜ 2 T 2 transition centred at 310 nm. Emission monitored via the SiF 41 D c With the signal-to noise ratio of the time-resolved spectra, we assume that a flat decay over the full range of the multi-channel analyser, 200 ns, implies t .ca. 500 ns. d ˜ 2 ˜ 2 Emission monitored via the GeF 1 4 D A 1 –A T 2 transition centred at 255 nm. e Notation for bent or quasi-linear states given in both C 2v and D `h symmetry.
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2). The decay from the a˜ 3 B 1 state of GeF2 is too long to measure with BESSY 1 and, in particular, there is no evidence for the short lifetime component that is observed for this state of SiF 2 . If this decay in SiF 2 is due to fast, non-radiative inter-system crossing, its absence in GeF 2 is surprising, given the predicted larger spin-orbit coupling matrix element ˜ 1 A states due to the larger linking the a˜ 3 B 1 and X 1 atomic spin-orbit coupling constant of the Ge atom.
4. PF 3 Fluorescence excitation spectra of PF 3 in the range 9–20 eV were recorded at Daresbury with a range of optical filters [4]. In the lower-energy range, on thermodynamic grounds fluorescence can only be due to the PF 2 ? radical. Emission from the parent ion of PF 3 is not observed. Transitions to the three lowest-energy bands in the Rydberg spectra between 9–13 eV show resolved vibrational structure (v2 5 458624 cm 21 ). They are assigned to transitions to the (8a 1 )21 4p, 5p and 6p Rydberg states of PF 3 , with fluorescence being due to valence transitions in the PF 2 ? radical. From a Franck–Condon analysis of the vibrational structure, we estimate that the FPF bond angle in PF 3 increases by ca. 14618 upon photoexcitation [10]. These Rydberg states of PF 3 and the ground state of PF 31 to which they converge, therefore have a bond angle of ca. 1128, remaining of pyramidal geometry. The barrier to planarity of the ground state of PF 1 3 is calculated to be 2.08 eV [10]. At excitation energies greater than 16 eV, fluorescence is due solely to PF. From the BESSY 1 results, four different decay channels are observed (Fig. 3): ˜ 2 A ( 2 P)–X ˜ 2 B fluorescence in the wide 1. PF 2 ? A 1 1 range 320–550 nm for VUV photon energies around 9.8 eV, ˜ ˜ and B ˜ 2 B –X ˜ 2 B fluorescence at ca. 2. PF 2 ? A–X, 2 1 300 nm for VUV energies around 11.0 eV, ˜ 2 B and E˜ 2 B ( 2 P)–A ˜ 2 A ( 2 P) 3. PF 2 ? C˜ 2 A 1 –X 1 1 1 fluorescence at ca. 222 and 325 nm, respectively, for VUV energies around 14.4 eV, 4. PF A 3 P–X 3 S 2 fluorescence between 300–380 nm for photon energies around 16.1 eV.
Fig. 3. Dispersed emission spectra for PF 3 photoexcited at (a) 9.8, (b) 11.0, (c) 14.4 and (d) 16.1 eV. The optical resolution is ca. 8 nm. A Jobin Yvon H2OVIS monochromator is used for (a) and (b), an H20UV for (c) and (d). No attempt has been made to allow for the variation of sensitivity of the detection system with wavelength, but the former instrument has more sensitivity at l .ca. 450 nm. Assignments of the main emission bands are given. Note that the band at 325 nm in (a) arises due to secondorder radiation from the primary monochromator. (Reproduced by permission from Ref. [4].)
The assignments in PF 2 ? are heavily dependent on recent ab initio calculations [11] on the geometries and energies of the valence electronic states of this species, and on what emissions are thermochemically ˜ 2 A and allowed [4]. The lifetimes of the bent C 1 2 2 ˜ quasi-linear E B 1 ( P) states of PF 2 ? are measured to be 14.7 and 7.9 ns, respectively (Table 2). The lifetimes of the other emitters are too long to measure at BESSY 1. This work constitutes the most extensive experimental study to date of the valence spectroscopy of the PF 2 ? radical.
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5. CCl 4 , SiCl 4 and GeCl 4 Fluorescence excitation spectra of these molecules have been recorded at Daresbury in the VUV energy range 9–25 eV with a resolution of ca. 0.06 eV. Resonant peaks only are observed with CCl 4 , both resonant and non-resonant peaks with SiCl 4 and GeCl 4 . All the Rydberg peaks of MCl 4 are assigned. With CCl 4 , fluorescence is due to CCl 2 at lowenergy and CCl at high energy. With SiCl 4 [GeCl 4 ], fluorescence is due to SiCl 2 [GeCl 2 ] at low-energy, SiCl 41 [GeCl 41 ] at intermediate energy, and Si [Ge] at high energy. Dispersed emission spectra have been recorded with an optical resolution of 4 nm at BESSY 1 at the energies of the peaks in the excitation spectra. Specifically, the five different decay channels observed are [5]:
˜ 1 B –X ˜ 1 A fluorescence from CCl 1. CCl 2 A 1 1 4 excited in the VUV energy range 9–12 eV, ˜ 1 A and A ˜ 1 B –X ˜ 1A 2. SiCl 2 / GeCl 2 a˜ 3 B 1 –X 1 1 1 fluorescence from SiCl 4 / GeCl 4 excited in the VUV range 9–14 eV ˜ 2 T –X ˜ 2 T and C ˜ 2 T –A ˜ 2T 3. SiCl 41 / GeCl 41 C 2 1 2 2 fluorescence from SiCl 4 / GeCl 4 for energies ˜ above the adiabatic ionisation energy of MCl 1 4 C 2 T2, 4. CCl A 2 D–X 2 P fluorescence from CCl 4 excited in the range 14–18 eV, 5. Si and Ge atomic lines from SiCl 4 / GeCl 4 excited in the range 19–25 eV. These assignments are confirmed by appropriate action spectra. Using single-bunch mode, lifetimes of all emitting states that fall in the range ca. 3–100 ns ˜ 1B have been measured. The results for SiCl 2 A 1 1 ˜ 2 1 ˜ (6769 ns), SiCl 4 C T 2 (37.560.4 ns) and GeCl 4 C 2 T 2 (6564 ns) confirm data from other experiments [5,9,12]. The lifetimes of the a˜ 3 B 1 states of both ˜ 1 B state of SiCl 2 and GeCl 2 , and that of the A 1 GeCl 2 are too long to measure. The value we measure for CCl A 2 D, 5362 ns, is the average lifetime for the v50, 1 and 2 levels of this state that lie below the barrier to predissociation in the calculated potential energy curve [13]. Lifetimes of several atomic emissions in Si and Ge are measured for
the first time. The decay from some of these excited states, surprisingly, shows bi-exponential behaviour [5]. Emissions in CCl 3 ?, SiCl 3 ? and GeCl 3 ? are not observed. Perhaps the most interesting aspect of this work concerns the photodissociation dynamics of the Rydberg states of MCl 4 . The MCl 2* products are formed by direct, one-step photodissociation of lowlying Rydberg states of MCl 4 , i.e. MCl 4 1 hn1 → MCl 4* → MCl *2 1 Cl *2 or 2Cl, MCl *2 → MCl 2 1 hn2
(3)
The thresholds for production of MCl *2 therefore relate to energies of Rydberg states of the parent molecule, often lying several eV above their thermochemical threshold. It is not possible to say whether Cl 2 or 2Cl are the other product(s). By contrast, the CCl*, Si* and Ge* products are probably formed by sequential, multi-step photodissociation of MCl *4 , e.g. SiCl 4 1 hn1 ( . 21.3 eV) → SiCl 4* → SiCl 3* 1 Cl → SiCl *2 1 2Cl, → SiCl* 1 3Cl → Si* ( 3 PJ ) 1 4Cl Si* → Si 1 hn2
(4)
The thresholds for production of CCl*, Si* and Ge* now correspond to the thermodynamic energies to form the emitting product with three (in the case of CCl*) or four (in the case of Si* / Ge*) chlorine atoms.
6. CHCl 3 , CFCl 3 and CBrCl 3 For these less symmetrical molecules, following VUV photoexcitation in the range 10–20 eV more fragmentation channels are possible since different bonds can break [6]. Thus, for CHCl 3 , resonant peaks in the range 10–13 eV are observed, with ˜ 1 B –X ˜ 1 A and emission being due to both CCl 2 A 1 1 1 1 ˜ ˜ CHCl A A0–X A9. At higher energies, more fragmentation occurs with emissions being observed in the three diatomic molecules CCl?, CH? and Cl 2 . Similar behaviour is observed with CFCl 3 . At low
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˜ ˜ and CFCl A ˜ 1 A0–X ˜ 1 A9 emisenergies, CCl 2 A–X sions dominate, with subtle variations in the branching ratio to these two channels as the VUV excitation energy changes. At higher energies, emissions are observed in CF?, CCl? and Cl 2 . A surprising result is that emission is only observed from the B 2 D state of CF?, and not from the lower-lying A 2 S 1 valence state. No parent ion emission is observed in these three CXCl 3 molecules. This work points to the future, an area we describe as high-energy photophysics. Knowledge of VUV photophysics above the first ionisation energy of a polyatomic molecule is poor. It is now clear that quantum yields for ionisation are not universally unity once the threshold for ionisation is reached. The phenomenon of multiple-bond breaking, not photoionisation, can be important at these higher energies. In the extreme, a molecule can fragment to its constituent atoms (e.g. Eq. (4)). There are many unsolved problems. For example, do the bonds break simultaneously or by a series of sequential bond cleavages? If the latter process dominates, what are the timescales? There are considerable advantages to using a single-photon, tunable source such as synchrotron radiation to study the dynamics of such reactions. First, the threshold for production of a fluorescing state of a fragment can be determined. In turn, this threshold can be related to the thermodynamics of the different product channels, and it may be possible to say what the other products of the photodissociation are. Second, it constitutes a cleaner way to excite a fragment radical than a fixed-energy multi-photon laser source where often the number of photons involved is unclear and the excitation energy may not be optimum. We are currently carrying out more such experiments on a range of CCl 2 -, CF 2 -, CF 3 -, CH 2 - and CH 3 -containing molecules.
Acknowledgements EPSRC, UK is thanked for for Research Studentships (KJB and DPS), Daresbury Laboratory for a
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CASE award (KJB). HB thanks the DFG, Germany for a Post-Doctoral Research Fellowship. The EU Human Capitol and Mobility programme (contract number ERBFMGE-CT-950031) and the British Council (ARC bilateral programme with Germany, contract number 707) are thanked for funding of the BESSY 1 part of this project.
References [1] R.P. Tuckett, Chem. Soc. Rev. 19 (1990) 439. [2] H. Biehl, K.J. Boyle, D.P. Seccombe, D.M. Smith, R.P. ¨ Tuckett, K.R. Yoxall, H. Baumgartel, H.W. Jochims, J. Chem. Phys. 107 (1997) 720. ¨ [3] K.J. Boyle, D.P. Seccombe, R.P. Tuckett, H. Baumgartel, H.W. Jochims, Chem. Phys. Letts. 294 (1998) 50. [4] H. Biehl, K.J. Boyle, D.P. Seccombe, R.P. Tuckett, H. ¨ Baumgartel, H.W. Jochims, J. Chem. Phys. 108 (1998) 857. [5] H. Biehl, K.J. Boyle, D.P. Seccombe, D.M. Smith, R.P. ¨ Tuckett, H. Baumgartel, H.W. Jochims, J. Elec. Spec. Rel. Phen. 97 (1998) 89. ¨ [6] D.P. Seccombe, R.P. Tuckett, H. Baumgartel, H.W. Jochims, Phys. Chem. Chem. Phys. (1999) in press. [7] e.g. V.M. Donnelly, D.L. Flamm, J. Appl. Phys. 51 (1980) 5273. [8] D.J. Clouthier, private communication. [9] D.M. Smith, R.P. Tuckett, K.R. Yoxall, K. Codling, P.A. Hatherly, J.F.M. Aarts, M. Stankiewicz, J. Chem. Phys. 101 (1994) 10559. [10] R.P. Tuckett, P.J. Knowles, Chem. Phys. Letts. 261 (1996) 486. [11] e.g. L. Latifzadeh, K. Balasubramanian, Chem. Phys. Letts. 228 (1994) 463. [12] M. Suzuki, N. Washida, G. Inoue, Chem. Phys. Letts. 131 (1986) 24. [13] M. Larsson, M.R.A. Blomberg, P.E.M. Siegbahn, Mol. Phys. 46 (1982) 365.
Vacuum-UV fluorescence spectroscopy of polyatomic molecules ¨ b , H.W. Jochims b R.P. Tuckett a , *, H. Biehl a , K.J. Boyle a , D.P. Seccombe a , H. Baumgartel a
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2 TT, UK b ¨ f ur ¨ Physikalisch Chemie, Freie Universitat ¨ Berlin, D-14195 Berlin, Germany Institut Received 4 August 1998
Abstract Experiments are described in which UV and visible fluorescence is observed following vacuum-UV photoexcitation of a range of polyatomic molecules in the gas phase. Tunable radiation in the energy range 8–25 eV from synchrotron sources at Daresbury, UK and BESSY 1, Germany is used as the photoexcitation source. Non-dispersed fluorescence excitation spectra and dispersed fluorescence spectra are recorded at Daresbury and BESSY 1, respectively. The experiments are sensitive to Rydberg states of molecules that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresces. Using single-bunch mode of BESSY 1, lifetimes of the emitters in the range ca. 3–100 ns are measured. Examples are taken from work either published or in press in SiF 4 and GeF 4 , PF 3 , MCl 4 (M5C, Si, Ge), and CXCl 3 (X5H, F, Br). 1999 Elsevier Science B.V. All rights reserved.
ABn 1 hv1 → (ABn )* → (ABn21 )* 1 B
1. Introduction In this second paper we describe experiments in which UV and visible fluorescence is observed following vacuum-UV excitation of a range of polyatomic molecules. The two groups comprising the authors have collaborated to study the photodissociation dynamics of both the Rydberg states of polyatomic molecules with four to six atoms and the valence states of their parent molecular ion. Specifically, our experiments are sensitive to those Rydberg states that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresce. If ABn represents a general polyatomic molecule, these two processes can be represented as:
*Corresponding author. Tel.: 144-121-414-425; fax: 144-121414-4403. E-mail address: [email protected] (R.P. Tuckett)
or (ABn22 )* 1 2B, (ABn21 )* or (ABn22 )* → ABn21 or ABn22 1 hn2 (1) and ABn 1 hv1 → (AB n1 )* 1 e 2 , 1 (AB 1 n )* → AB n 1 hv 2
(2)
hn1 and hn2 represent photons in the vacuum ultraviolet (VUV, 50, l1 ,150 nm or 8,E1 ,25 eV) and ultraviolet / visible (200, l2 ,700 nm), respectively. Processes 1 and 2 lead to resonant and non-resonant peaks, respectively, in the VUV fluorescence excitation spectrum of ABS. Process 1 is termed resonant, because the VUV photon must populate resonantly the Rydberg state of ABn . Process 2 is termed nonresonant because, as in photoelectron spectroscopy, the signal is still observed for photon energies well
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00419-8
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in excess of the threshold to produce (AB 1 n )* because the electron can remove the excess energy. These two processes lead to different peak shapes in the fluorescence excitation spectrum [1]. Using synchrotron radiation, it has been possible to populate, state-selectively and often at vibrational resolution, Rydberg states and valence molecular ion states of a range of polyatomic molecules in the energy range ca. 8–25 eV for such studies. In some circumstances we have been able to comment on the detailed photodissociation dynamics of Rydberg states. This paper summarises the important results for SiF 4 [2] and GeF 4 [3], PF 3 [4], MCl 4 (M5C, Si, Ge) [5], and CXCl 3 (X5H, F, Br) [6]. From an applied scientific viewpoint, many of these molecules are used in the plasma etching of integrated circuits, and our results should be of use to scientists studying such processes. In RF discharges, neutral free radicals and molecular ions are created in an ill-defined way by low-energy electrons whose energies span the wide range ca. 3–50 eV. In this project, we use the more controllable method of photoexcitation with photons spanning the same energy range as the plasma electrons. Our results could have diagnostic applications in the monitoring of species present in plasmas.
Fig. 1. Schematic of the non-dispersed fluorescence excitation apparatus used at Daresbury.
of the electronics limits the lifetimes that can be measured to ca. 3–100 ns. Three characteristics of the Daresbury experiment are important. First, flux normalisation to the VUV
2. Experimental Experiments are performed at synchrotron sources at Daresbury, UK and BESSY 1, Germany. Full details are given elsewhere [2,4]. Non-dispersed, fluorescence excitation spectra are recorded at Daresbury (Fig. 1) using either a 1 m Seya-Namioka (range 8–35 eV, best resolution 0.05 nm) or a 5 m normal-incidence McPherson monochromator (range 8–30 eV, best resolution 0.01 nm) attached to the 2 GeV electron storage ring as the primary source of tunable VUV radiation. Dispersed fluorescence and VUV action spectra are recorded at BESSY 1 (Fig. 2), using a 1.5 m normal-incidence monochromator (range 7–25 eV, optimum resolution 0.03 nm) attached to the 800 MeV electron storage ring to monochromatise the radiation. Using the singlebunch mode of BESSY 1 (20 ps pulses every 208 ns), we measure the lifetimes of the emitting states, but the timing profile of the source and response time
Fig. 2. Schematic of the dispersed fluorescence apparatus used at BESSY 1.
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photon beam is performed in situ using a sodium salicylate window and a photomultiplier tube. Second, the use of optical filters can isolate the range of wavelengths over which undispersed fluorescence is detected by the photon-counting EMI 9813 QB photomultiplier tube (range 190–650 nm), in effect affording a low-resolution action spectrum. Third, the experiment gives information on the spectroscopy of the Rydberg states of the polyatomic molecule, valence states of the parent ion that decay radiatively, and fluorescence excitation functions. A limited amount of data only is obtained about the spectroscopy of the emitting species. There are also three important characteristics of the BESSY 1 experiment. First, it is difficult to flux-normalise fluorescence excitation spectra in situ to the VUV photon beam. Second, a 0.2 m secondary monochromator (Jobin Yvon H2OVIS) with no entrance slit and an exit slit of 1–2 mm, giving a resolution of ca. 4–8 nm, disperses the induced fluorescence. Experiments can disperse the fluorescence after excitation at a fixed (primary) VUV energy, giving low-resolution electronic spectra of the emitting species. Action spectra, where the VUV energy is scanned for detection at a known secondary wavelength, can also be obtained. Third, the ability to disperse the induced fluorescence, determine the wavelength of the emission and hence define the emitter, makes this an ideal apparatus for measuring
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lifetimes of fluorescing states of neutral free radicals and molecular ions. This is particularly true when more than one emitter is produced at any VUV photon energy.
3. SiF 4 and GeF 4 The plasma dry etching of silicon wafers by fluorine-containing gases (e.g. CF 4 , C 2 F 6 ) has applications to the fabrication of microelectronic devices. The role of the SiF 2 and SiF 3 ? radicals in the etching process is known to be important [7], hence a knowledge of the electronic spectroscopy of these species is needed. To date, the electronic spectroscopy of SiF 3 ? is very poorly understood, and there are major gaps in our knowledge of the excited states of SiF 2 . Both resonant and non-resonant peaks are observed in the VUV fluorescence excitation spectrum of SiF 4 between 10 and 25 eV [2]. Transitions to low-lying Rydberg states in this range that photodissociate to a fluorescing state of a fragment radical have all been assigned (Table 1). Fluorescence has been observed as valence transitions in SiF 3 ?, in SiF 2 , and, especially strongly, in the parent ion SiF 41 . Fluorescence from excited states of the SiF radical is thermochemically forbidden, and has not been observed. From the dispersed spectra obtained at
Table 1 Peak positions and assignments from fluorescence excitation spectroscopy of the Rydberg and ionic states of SiF 4 in the range 10–25 eV, and assignments of the fluorescing fragments and ions E / eV a 13.0 13.9 14.8 15.95 18.1 19.45 .21.5 c
Assignment 21
(IE 2 E) / eV
(1t 1 ) 3p 3.5 or (3t 2 )21 3s 4.4 (3t 2 )21 3p 3.5 (1t 1 )21 3d 1.7 or (1e)21 3p 3.2 (3t 2 )21 3d 1.45 or (3t 2 )21 4p 1.45 (2t 2 )21 4p 1.36 (2a 1 )21 4s 2.1 2 ˜ 2 (2a 1 )21 →SiF 1 4 D A 1 1e
(n2d )
db
Emission range / nm
Emitter
1.97 1.76 1.97 2.83 2.06 3.06 3.06 3.16 2.54
1.03 1.24 1.03 0.17 0.94 20.06 0.94 0.84 1.46
ca. 380– 800
SiF 3 ?*
ca. 380–800 ca. 380– 800
SiF 3 ?* SiF 3 ?*
ca. 360–440
SiF 2 a˜ 3 B 1
ca. 220–280 ca. 220–280 ca. 280–350
˜ 1B 1 SiF 2 A ˜ 1B 1 SiF 2 A 1 ˜ 2 SiF 4 D A 1
˜ 2 A 1 state of SiF 1 The effects of second-order radiation from the primary monochromator producing emission from the D 4 at excitation energies less than 21.5 eV [2] are ignored in this table. b Quantum defect, d, defined by the equation E5IE2[R H /(n2d )2 ], where R H is the Rydberg constant and n is the principal quantum number of the Rydberg orbital. Calculated using the appropriate vertical ionisation energies (IE) for SiF 4 from threshold photoelectron spectroscopy [9]. c Threshold for fluorescence, not peak position. a
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BESSY 1, four different decay channels are observed:
1. SiF 3 ? fluorescence in the range 380–800 nm for VUV photon energies around 13.0 eV, ˜ 1 A phosphorescence from 360–440 2. SiF 2 a˜ 3 B 1 –X 1 nm for VUV energies in the range 15.2–18.0 eV, ˜ 1 B –X ˜ 1 A fluorescence from 210–270 3. SiF 2 A 1 1 nm for VUV energies in the range 17.0–20.0 eV, ˜ 2 A state of SiF 1 4. intense emission from the D 1 4 predominantly in the range 280–350 nm for VUV energies greater than the adiabatic ionisation energy of this state, 21.5 eV. The lifetimes of the unassigned emitting state in ˜ 1 B state of SiF and the D ˜ 2 A state of SiF 3 ?, the A 1 2 1 SiF 41 have been measured to be 3.9, 11.2 and 9.16 ns, respectively (Table 2) [2]. The SiF 3 ? result is new, the symmetry of the emitting state is unknown, and it is not even clear whether emission is occurring to the ground or an excited state of the radical. This work has highlighted the urgent need for high quality ab initio calculations on the energies and geometries
of excited states of SiF 3 ?. There is one surprising 3 lifetime result. The decay from the a˜ B 1 state of SiF 2 has a fast component of 3.3 ns (Table 2) [3]. We conclude that the radiative lifetime of this state is either as low as 3.3 ns or too high (t .ca. 500 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for spin-forbidden phosphor˜ 1 A ground state, the 3.3 ns escence to the X 1 component could be the lifetime of inter-system crossing from higher vibrational levels of the a˜ 3 B 1 state of SiF 2 into its ground state [2,8]. With GeF 4 , only three decay channels are ob3 ˜ 1 A phosphorescence, served [3]: GeF 2 a˜ B 1 –X 1 1 1 ˜ ˜ GeF 2 A B 1 –X A 1 fluorescence, and emission from ˜ 2 A state of GeF 1 . There is no analagous the D 1 4 emission in the GeF 3 ? radical. With SiF 4 there is no evidence whether SiF *2 forms with F 2 or 2F as the other product(s) of SiF *4 photodissociation. With GeF 4 , however, there is strong evidence from ther˜ 1 B states of mochemistry that both the a˜ 3 B 1 and A 1 GeF 2 form via a one-step mechanism with F 2 , and ˜ 1B not 2F, as the other product. Lifetimes of the A 1 ˜ 2 A state of GeF 1 are state of GeF 2 and the D 1 4 measured to be 9.3 and 5.02 ns, respectively (Table
Table 2 Lifetimes of emission bands observed from VUV excitation of SiF 4 (top), GeF 4 (middle) and PF 3 (bottom) in the range 10–25 eV E1 / eV
l2 / nm (68 nm)
t / ns
Reduced x 2
Emitter
13.0 15.9 18.1 21.8
480 400 225 310
3.960.7 3.360.4 11.261.5 9.1660.02
1.71 1.50 2.01 1.33
SiF 3 ?* SiF 2 a˜ 3 B 1 a ˜ 1B 1 SiF 2 A 1 ˜ 2 SiF 4 D A 1 b
14.8 15.9 21.4
340 235 255
Flat decay c 9.360.1 5.0260.01
2.67 1.07
GeF 2 a˜ 3 B 1 ˜ 1B 1 GeF 2 A 2 ˜ GeF 1 D A1 d 4
9.8 11.0 14.4 14.4 16.1
450 300 222 325 322
Flat decay Flat decay 14.760.1 7.960.1 Flat decay
1.55 1.70
˜ 2 A 1 ( 2 P)e PF 2 ? A ˜ 2B 2 PF 2 ? B ˜ 2A 1 PF 2 ? C ˜ PF 2 ? E 2 B 1 ( 2 P)e PF A 3 P
a Almost certainly, this value is the lifetime for inter-system crossing of vibrational levels of SiF 2 a˜ 3 B 1 to high vibrational levels of the electronic ground state, not the radiative lifetime of the a˜ 3 B 1 state. b ˜ 2 A 1 –A ˜ 2 T 2 transition centred at 310 nm. Emission monitored via the SiF 41 D c With the signal-to noise ratio of the time-resolved spectra, we assume that a flat decay over the full range of the multi-channel analyser, 200 ns, implies t .ca. 500 ns. d ˜ 2 ˜ 2 Emission monitored via the GeF 1 4 D A 1 –A T 2 transition centred at 255 nm. e Notation for bent or quasi-linear states given in both C 2v and D `h symmetry.
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2). The decay from the a˜ 3 B 1 state of GeF2 is too long to measure with BESSY 1 and, in particular, there is no evidence for the short lifetime component that is observed for this state of SiF 2 . If this decay in SiF 2 is due to fast, non-radiative inter-system crossing, its absence in GeF 2 is surprising, given the predicted larger spin-orbit coupling matrix element ˜ 1 A states due to the larger linking the a˜ 3 B 1 and X 1 atomic spin-orbit coupling constant of the Ge atom.
4. PF 3 Fluorescence excitation spectra of PF 3 in the range 9–20 eV were recorded at Daresbury with a range of optical filters [4]. In the lower-energy range, on thermodynamic grounds fluorescence can only be due to the PF 2 ? radical. Emission from the parent ion of PF 3 is not observed. Transitions to the three lowest-energy bands in the Rydberg spectra between 9–13 eV show resolved vibrational structure (v2 5 458624 cm 21 ). They are assigned to transitions to the (8a 1 )21 4p, 5p and 6p Rydberg states of PF 3 , with fluorescence being due to valence transitions in the PF 2 ? radical. From a Franck–Condon analysis of the vibrational structure, we estimate that the FPF bond angle in PF 3 increases by ca. 14618 upon photoexcitation [10]. These Rydberg states of PF 3 and the ground state of PF 31 to which they converge, therefore have a bond angle of ca. 1128, remaining of pyramidal geometry. The barrier to planarity of the ground state of PF 1 3 is calculated to be 2.08 eV [10]. At excitation energies greater than 16 eV, fluorescence is due solely to PF. From the BESSY 1 results, four different decay channels are observed (Fig. 3): ˜ 2 A ( 2 P)–X ˜ 2 B fluorescence in the wide 1. PF 2 ? A 1 1 range 320–550 nm for VUV photon energies around 9.8 eV, ˜ ˜ and B ˜ 2 B –X ˜ 2 B fluorescence at ca. 2. PF 2 ? A–X, 2 1 300 nm for VUV energies around 11.0 eV, ˜ 2 B and E˜ 2 B ( 2 P)–A ˜ 2 A ( 2 P) 3. PF 2 ? C˜ 2 A 1 –X 1 1 1 fluorescence at ca. 222 and 325 nm, respectively, for VUV energies around 14.4 eV, 4. PF A 3 P–X 3 S 2 fluorescence between 300–380 nm for photon energies around 16.1 eV.
Fig. 3. Dispersed emission spectra for PF 3 photoexcited at (a) 9.8, (b) 11.0, (c) 14.4 and (d) 16.1 eV. The optical resolution is ca. 8 nm. A Jobin Yvon H2OVIS monochromator is used for (a) and (b), an H20UV for (c) and (d). No attempt has been made to allow for the variation of sensitivity of the detection system with wavelength, but the former instrument has more sensitivity at l .ca. 450 nm. Assignments of the main emission bands are given. Note that the band at 325 nm in (a) arises due to secondorder radiation from the primary monochromator. (Reproduced by permission from Ref. [4].)
The assignments in PF 2 ? are heavily dependent on recent ab initio calculations [11] on the geometries and energies of the valence electronic states of this species, and on what emissions are thermochemically ˜ 2 A and allowed [4]. The lifetimes of the bent C 1 2 2 ˜ quasi-linear E B 1 ( P) states of PF 2 ? are measured to be 14.7 and 7.9 ns, respectively (Table 2). The lifetimes of the other emitters are too long to measure at BESSY 1. This work constitutes the most extensive experimental study to date of the valence spectroscopy of the PF 2 ? radical.
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5. CCl 4 , SiCl 4 and GeCl 4 Fluorescence excitation spectra of these molecules have been recorded at Daresbury in the VUV energy range 9–25 eV with a resolution of ca. 0.06 eV. Resonant peaks only are observed with CCl 4 , both resonant and non-resonant peaks with SiCl 4 and GeCl 4 . All the Rydberg peaks of MCl 4 are assigned. With CCl 4 , fluorescence is due to CCl 2 at lowenergy and CCl at high energy. With SiCl 4 [GeCl 4 ], fluorescence is due to SiCl 2 [GeCl 2 ] at low-energy, SiCl 41 [GeCl 41 ] at intermediate energy, and Si [Ge] at high energy. Dispersed emission spectra have been recorded with an optical resolution of 4 nm at BESSY 1 at the energies of the peaks in the excitation spectra. Specifically, the five different decay channels observed are [5]:
˜ 1 B –X ˜ 1 A fluorescence from CCl 1. CCl 2 A 1 1 4 excited in the VUV energy range 9–12 eV, ˜ 1 A and A ˜ 1 B –X ˜ 1A 2. SiCl 2 / GeCl 2 a˜ 3 B 1 –X 1 1 1 fluorescence from SiCl 4 / GeCl 4 excited in the VUV range 9–14 eV ˜ 2 T –X ˜ 2 T and C ˜ 2 T –A ˜ 2T 3. SiCl 41 / GeCl 41 C 2 1 2 2 fluorescence from SiCl 4 / GeCl 4 for energies ˜ above the adiabatic ionisation energy of MCl 1 4 C 2 T2, 4. CCl A 2 D–X 2 P fluorescence from CCl 4 excited in the range 14–18 eV, 5. Si and Ge atomic lines from SiCl 4 / GeCl 4 excited in the range 19–25 eV. These assignments are confirmed by appropriate action spectra. Using single-bunch mode, lifetimes of all emitting states that fall in the range ca. 3–100 ns ˜ 1B have been measured. The results for SiCl 2 A 1 1 ˜ 2 1 ˜ (6769 ns), SiCl 4 C T 2 (37.560.4 ns) and GeCl 4 C 2 T 2 (6564 ns) confirm data from other experiments [5,9,12]. The lifetimes of the a˜ 3 B 1 states of both ˜ 1 B state of SiCl 2 and GeCl 2 , and that of the A 1 GeCl 2 are too long to measure. The value we measure for CCl A 2 D, 5362 ns, is the average lifetime for the v50, 1 and 2 levels of this state that lie below the barrier to predissociation in the calculated potential energy curve [13]. Lifetimes of several atomic emissions in Si and Ge are measured for
the first time. The decay from some of these excited states, surprisingly, shows bi-exponential behaviour [5]. Emissions in CCl 3 ?, SiCl 3 ? and GeCl 3 ? are not observed. Perhaps the most interesting aspect of this work concerns the photodissociation dynamics of the Rydberg states of MCl 4 . The MCl 2* products are formed by direct, one-step photodissociation of lowlying Rydberg states of MCl 4 , i.e. MCl 4 1 hn1 → MCl 4* → MCl *2 1 Cl *2 or 2Cl, MCl *2 → MCl 2 1 hn2
(3)
The thresholds for production of MCl *2 therefore relate to energies of Rydberg states of the parent molecule, often lying several eV above their thermochemical threshold. It is not possible to say whether Cl 2 or 2Cl are the other product(s). By contrast, the CCl*, Si* and Ge* products are probably formed by sequential, multi-step photodissociation of MCl *4 , e.g. SiCl 4 1 hn1 ( . 21.3 eV) → SiCl 4* → SiCl 3* 1 Cl → SiCl *2 1 2Cl, → SiCl* 1 3Cl → Si* ( 3 PJ ) 1 4Cl Si* → Si 1 hn2
(4)
The thresholds for production of CCl*, Si* and Ge* now correspond to the thermodynamic energies to form the emitting product with three (in the case of CCl*) or four (in the case of Si* / Ge*) chlorine atoms.
6. CHCl 3 , CFCl 3 and CBrCl 3 For these less symmetrical molecules, following VUV photoexcitation in the range 10–20 eV more fragmentation channels are possible since different bonds can break [6]. Thus, for CHCl 3 , resonant peaks in the range 10–13 eV are observed, with ˜ 1 B –X ˜ 1 A and emission being due to both CCl 2 A 1 1 1 1 ˜ ˜ CHCl A A0–X A9. At higher energies, more fragmentation occurs with emissions being observed in the three diatomic molecules CCl?, CH? and Cl 2 . Similar behaviour is observed with CFCl 3 . At low
R.P. Tuckett et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 101 – 107
˜ ˜ and CFCl A ˜ 1 A0–X ˜ 1 A9 emisenergies, CCl 2 A–X sions dominate, with subtle variations in the branching ratio to these two channels as the VUV excitation energy changes. At higher energies, emissions are observed in CF?, CCl? and Cl 2 . A surprising result is that emission is only observed from the B 2 D state of CF?, and not from the lower-lying A 2 S 1 valence state. No parent ion emission is observed in these three CXCl 3 molecules. This work points to the future, an area we describe as high-energy photophysics. Knowledge of VUV photophysics above the first ionisation energy of a polyatomic molecule is poor. It is now clear that quantum yields for ionisation are not universally unity once the threshold for ionisation is reached. The phenomenon of multiple-bond breaking, not photoionisation, can be important at these higher energies. In the extreme, a molecule can fragment to its constituent atoms (e.g. Eq. (4)). There are many unsolved problems. For example, do the bonds break simultaneously or by a series of sequential bond cleavages? If the latter process dominates, what are the timescales? There are considerable advantages to using a single-photon, tunable source such as synchrotron radiation to study the dynamics of such reactions. First, the threshold for production of a fluorescing state of a fragment can be determined. In turn, this threshold can be related to the thermodynamics of the different product channels, and it may be possible to say what the other products of the photodissociation are. Second, it constitutes a cleaner way to excite a fragment radical than a fixed-energy multi-photon laser source where often the number of photons involved is unclear and the excitation energy may not be optimum. We are currently carrying out more such experiments on a range of CCl 2 -, CF 2 -, CF 3 -, CH 2 - and CH 3 -containing molecules.
Acknowledgements EPSRC, UK is thanked for for Research Studentships (KJB and DPS), Daresbury Laboratory for a
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CASE award (KJB). HB thanks the DFG, Germany for a Post-Doctoral Research Fellowship. The EU Human Capitol and Mobility programme (contract number ERBFMGE-CT-950031) and the British Council (ARC bilateral programme with Germany, contract number 707) are thanked for funding of the BESSY 1 part of this project.
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