The C̃–X̃ electronic spectrum of the SrNC free radical: a jet-cooled investigation

The C̃–X̃ electronic spectrum of the SrNC free radical: a jet-cooled investigation

22 December 2000 Chemical Physics Letters 332 (2000) 303±307 www.elsevier.nl/locate/cplett ~ X ~ electronic spectrum of the SrNC free radical: The ...

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22 December 2000

Chemical Physics Letters 332 (2000) 303±307

www.elsevier.nl/locate/cplett

~ X ~ electronic spectrum of the SrNC free radical: The C± a jet-cooled investigation Gregory M. Greetham, Andrew M. Ellis * Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK Received 19 September 2000; in ®nal form 13 October 2000

Abstract The quasi-continuous band system of the SrNC free radical ®rst observed many years ago by Pasternack and ~ X ~ electronic transition, has been subjected to a jet-cooled laser-induced ¯uorescence Dagdigian, and attributed to the C± study. This new work reveals abundant vibrational structure that was largely hidden in the earlier study. The spectrum is not amenable to a simple vibrational analysis, presumably because of extensive perturbations. However, the fact that the structure spans more than 3500 cmÿ1 reveals that SrNC undergoes a major geometry change on electronic excitation. Since it is known to be linear in its ground electronic state (2 R‡ ), the activity of the bending vibration in the spectrum suggests that SrNC adopts a nonlinear equilibrium geometry on excitation. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction The ®rst spectroscopic observation of the monoisocyanides of the alkaline earth metals Ca, Sr and Ba, was reported by Pasternack and Dagdigian [1]. The spectra obtained, which were recorded using laser-induced ¯uorescence (LIF), ~ X, ~ X ~ B± ~ X, ~ and C± ~ regions of these spanned the A± ~ 2 P±X ~ 2 R‡ molecules. Since that early work, the A system has been extensively studied at both lowand high-resolution [2±5], and there has also been ~ 2 R‡ system [5]. ~ 2 R‡ ±X some further work on the B However, the study by Pasternack and Dagdigian remains the sole previous published data on the ~ X ~ systems of the alkaline earth monoisocyaC± nides.

*

Corresponding author. Fax: +44-116-252-3789. E-mail address: [email protected] (A.M. Ellis).

Pasternack and Dagdigian generated these molecules by reacting an e€usive atomic metal beam, produced by oven evaporation of the solid metal, with gaseous BrCN. The spectra for the ~ X ~ systems were broad and largely featureless, C± except for SrNC where there was some evidence of weak resolved structure on top of an intense quasicontinuous background signal. However, there was no pattern to this structure, and therefore speci®c band assignments were not attempted. The ~ state was assumed to be a 2 P state, which could C therefore undergo Renner±Teller activity through coupling of the electronic and vibrational angular ~ X ~ systems was momenta. The breadth of the C± attributed, at least in part, to activity in the lowfrequency bending vibration brought about by the Renner±Teller e€ect. ~ X ~ system In this work we have revisited the C± of SrNC and recorded the ®rst jet-cooled spectrum for this electronic transition. The quasi-continuous

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background obtained in the earlier work is completely absent in this jet-cooled study and abundant vibrational structure is clearly observed. However, the spectrum is not easy to interpret and few bands succumb to a simple vibrational analysis. Nevertheless, we present evidence that the vibrational structure is inconsistent with a transition in which the molecule is linear at equilibrium in both upper and lower electronic states. Instead, since SrNC is known to be linear in its ground electronic state, we propose that it adopts a nonlinear equilibrium structure in the excited state. This ties in with recent work from our laboratory on higher electronic states of this molecule, which also found a preference for nonlinear structures [6]. The new spectra of SrNC are reported and discussed in this Letter.

Fig. 1. Jet-cooled laser-induced ¯uorescence excitation spectrum of SrNC. Also shown are bands due to SrCCH, which are marked with arrows. The scan range covers four laser dyes and no attempt has been made to correct for variations of dye laser output as a function of wavelength. Consequently, the relative band intensities are not a true re¯ection of the relative transition probabilities. Modes m2 and m3 are the Sr±N stretching and bending modes, respectively.

2. Experimental Full details of the experimental procedure and apparatus have been given elsewhere [6,7]. Brie¯y, SrNC was prepared by laser ablation of a strontium target in the presence of acetonitrile (CH3 CN) prior to expansion into vacuum. Electronic spectra were then recorded using either LIF or resonance-enhanced multiphoton ionisation (REMPI) spectroscopy. The latter proved particularly useful in con®rming the identity of the spectral carrier, since spectra could be recorded mass selectively using a time-of-¯ight mass spectrometer. For the experiments described here the ion signal was gated at the m=e ˆ 114 mass channel, which corresponds to the most abundant SrNC isotopomer, 88 SrNC. The resolution in these experiments was limited primarily by the linewidth of the tunable dye laser, which was 0.3 cmÿ1 . Consequently, although vibrational structure could be resolved, no rotational structure was extracted in the present work. 3. Results and discussion 3.1. Spectral assignments A jet-cooled laser excitation spectrum spanning 21 000±25 000 cmÿ1 is shown in Fig. 1. Abundant

structure is seen across this spectrum. A number of relatively sharp peaks appear at the low wavenumber end of the spectrum, all located with arrows in Fig. 1. As reported elsewhere [8], these are ~ 0 2 D±X ~ 2 R‡ transition of due to the `forbidden' B SrCCH. All of the remaining bands, the majority of which are much broader than the SrCCH peaks, are attributed to SrNC. To con®rm the spectral carrier, we recorded mass-selective REMPI spectra. At the high wavenumber end of the spectrum (>23 000 cmÿ1 ) it proved possible to record one-colour …1 ‡ 1† spectra but at lower wavenumbers only two-colour (1 ‡ 10 ) experiments yielded any signal in the SrNC‡ channel. Fig. 2 compares a portion of the 1 ‡ 10 REMPI spectrum with the corresponding LIF spectrum, showing that the same features are obtained in both cases. These experiments con®rmed that the spectral carrier has a mass consistent with SrNC, although SrCN is also a possibility (see discussion later). The loss of signal in the 1 ‡ 1 experiments below 23 000 cmÿ1 is presumably due to the two-photon energy approaching or falling below the lowest ionization threshold of SrNC. This therefore indicates that the ®rst ionization energy is 65.7 eV. Since the electron being removed is from a largely metallocalised orbital [9], similar ionization energies

G.M. Greetham, A.M. Ellis / Chemical Physics Letters 332 (2000) 303±307

Fig. 2. Comparison of the assigned portion of the LIF excitation spectrum with the corresponding 1 ‡ 10 REMPI spectrum. For the REMPI spectrum the signal from the time-of-¯ight mass spectrometer was gated at mass 114, which corresponds to the most abundant isotopomer of SrNC …88 SrNC†. The ionizing photon was generated by frequency doubling the dye laser output and allowing both the fundamental and the doubled output to enter the spectrometer. Although the signal/noise ratio is worse in the REMPI spectrum, all the bands attributed to SrNC in the LIF spectrum are also clearly seen in the REMPI spectrum.

would be expected for a range of strontium-containing free radicals. Only SrF and SrOH have been subjected to previous ionization energy measurements. Hildenbrand carried out an electron impact mass spectrometric study of the alkaline earth monohalides and obtained 5:0 0:3 eV for the appearance potential of SrF‡ [10], while a similar study on the alkaline earth monohydroxides by Murad yielded 5:1  0:2 eV for SrOH‡ [11]. These values are in line with our estimated upper limit for the ionization energy of SrNC. The onset of SrNC signal in Fig. 1 occurs with the relatively strong band at 21 580 cmÿ1 . No discernible SrNC features were seen to the red of this strong band and so consequently, given its intensity relative to other SrNC bands in the spectrum, a hot band assignment can be ruled out (see also the comments at the end of this section). We therefore assign the 21 580 cmÿ1 band as the ~ X ~ electronic system. origin (000 ) transition of the C± The next SrNC band is centered at 21 917 cmÿ1 , which is 337 cmÿ1 to the blue of the origin. This

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separation is close to the known Sr±NC stretching ~ 2 R‡ state, measured as …m3 † fundamental in the X ÿ1 338 cm by Lanzisera and Andrews [12] in a matrix isolation infrared study. Consequently, this band is tentatively assigned to the 310 transition. Moving to higher energy, the ®rm assignment of bands becomes a much more dicult prospect. It is clear that the Sr±NC stretch cannot be the sole active vibrational mode. The C  N stretch cannot play a signi®cant role since it will possess a frequency in excess of 2000 cmÿ1 . The only other mode of SrNC that could account for the density of vibrational structure is the bending vibration, m2 , which is likely to have a lower frequency than the Sr±NC stretch. However, if SrNC remains linear at equilibrium in the excited electronic state(s) then little Franck±Condon activity would be expected in the bending vibration. Although the Renner±Teller e€ect can induce some intensity in Franck±Condon forbidden transitions involving the bending mode [13,14], this alone could not account for the extensive structure seen in Fig. 1. Consequently, in order to account for the abundant structure observed, which spans more than 3500 cmÿ1 , we propose that SrNC deviates from linearity in the excited state. Armed even with this assumption, speci®c assignments of bands are dicult to make. The ®rst band to the blue of the 310 transition, that at 22 092 cmÿ1 , could be the 210 310 combination band and is labelled as such in Fig. 1. This would yield 175 cmÿ1 for the bending fundamental in the excited electronic state. However, given this assignment it is surprising that there is no trace of the 210 band lying between the 000 and 310 bands. Another diculty is that regular progressions in the bending mode cannot be identi®ed in the spectrum beyond the ®rst couple of members. Thus, although it seems likely that both the stretching and bending modes are active in the electronic spectrum, most of the bands do not conform to a simple vibrational analysis. To close this section, we point out that some experiments were also attempted under softer jet expansion conditions in order to obtain warmer spectra. The relative intensities of all the SrNC bands seen in Fig. 1 were unchanged under these new conditions. However, an additional weak

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peak, presumably arising from a hot band transition, appeared some 70 cmÿ1 to the red of each strong band. The most likely assignment for these hot bands is that they correspond to excitation of the bending fundamental in the ground electronic state, suggesting that the frequency of this mode is substantially reduced from that in the excited electronic state. 3.2. Bandwidths A notable feature of the spectrum in Fig. 1 is the considerable variation in widths of the SrNC bands, many of which are relatively broad. This is especially evident when compared with those of SrCCH in the same spectrum. The narrowest SrNC bands are mainly located towards the blue end of the spectrum. In contrast the ®rst ®ve bands are very broad, with full widths at half maximum averaging 35 cmÿ1 . Since the bandwidths vary substantially across the spectrum, the sample temperature cannot explain why some are particularly broad. Lifetime broadening can also be ruled out, since we have measured the excited state radiative lifetime for most of the upper state energy levels and obtained an approximate value of 80  20 ns in each case. This lifetime is the same, within the error limits, as the value of 104:4  6:3 ns previously determined by Pasternack and Dagdigian [1]. An alternative explanation for the large widths of many bands is that they arise from distinct but overlapping vibronic transitions. Ideally, these separate transitions could be distinguished in higher resolution scans. We attempted a number of careful scans of each band using small wavenumber steps but unfortunately, because of the linewidth of the dye laser (0.3 cmÿ1 ), distinct bands could not be di€erentiated from unresolved rotational contours. Nevertheless, the presence of overlapping transitions seems the most plausible explanation for the large widths of many of the SrNC bands. 3.3. Excited electronic state(s) ~ X ~ transition of SrNC can The position of the C± be predicted by analogy with the corresponding

transitions in SrF, SrCl, and SrOH. For the last three molecules the third excited electronic state is known to be a 2 P state. As we have argued in recent work [6], there appears to be a strong correlation between the energies of the electronic states of these molecules and the electron anities of the ligand attached to the metal atom. This is reasonable given the highly ionic character expected for the metal±ligand bonding in these molecules. In contrast to other known electronic transi~ X ~ tions of these molecules, the energy of the C± transition is reduced as the electron anity of the ligand is increased. We attribute this to the re~ states, in which verse-polarised character of the C the unpaired electron resides in a p orbital oriented towards rather than away from the ligand as in other nearby electronic states [7]. Consequently, a high ligand electron anity will tend to stabilise the excited electronic state and therefore red-shift ~ X ~ transition. The correlation with other the C± strontium-containing molecules leads us to predict ~ X ~ transition of SrNC will be found that the C± at <25 000 cmÿ1 , which is consistent with the position of the observed band system. However, the assignment to a 2 P±2 R‡ transition is clearly at odds with the observed vibrational structure. As indicated earlier, we propose that a nonlinear structure is preferred in the excited state. Consequently, the reverse-polarised 2 P excited state that would be obtained if the molecule were linear will be resolved into two distinct electronic states, a 2 A0 state and a 2 A00 state, in SrNC. This separation into two excited electronic states, both of which are accessible in fully allowed ~ 2 R‡ state, may also explain transitions from the X why some of the bands appear to consist of overlapping transitions. 3.4. SrNC or SrCN? It has been assumed so far that the spectral carrier is SrNC. However, an alternative possibility is that the other conformer, SrCN, is responsible. Indeed in the original work of Pasternack and Dagdigian [1] the spectral carrier was attributed to SrCN. Ab initio calculations [15,16], supported by high-resolution spectra [3,4,17±21],

G.M. Greetham, A.M. Ellis / Chemical Physics Letters 332 (2000) 303±307

have since established with certainty that the global minimum on the ground state potential energy surface corresponds to the isocyanide for all alkaline earth metals. Bauschlischer et al. found that for Ca and Ba (Sr was not considered) there is no energy barrier between the linear MCN and MNC structures, implying that the former is a transition state on the ground state surface [15]. However, more detailed calculations for Ca by Nanbu et al. [16] did ®nd a potential barrier between the two conformers, showing that both correspond to potential energy minima. Furthermore, both MgNC and MgCN have been independently identi®ed in millimeter wave spectra [20,21]. Since it is likely that SrNC/SrCN behave similarly, the spectrum we observe could conceivably arise from transitions in SrCN rather than SrNC. The current data do not allow the two cases to be distinguished, and it will require further work, most importantly much higher-resolution spectra, to establish which conformer is responsible. 4. Conclusions ~ X ~ A jet-cooled electronic spectrum in the C± region of SrNC has been reported for the ®rst time. Mass-resolved REMPI spectra show that the spectral carrier has a mass consistent with SrNC, although the metastable conformer, SrCN, is also a candidate. The vibrationally resolved spectrum shows extensive structure which does not yield to a simple vibrational analysis. However, there appears to be substantial activity in both the Sr± (NC) stretching mode and the bending mode. We have interpreted the activity of the latter as arising from a change of equilibrium geometry, i.e. the molecule adopts a nonlinear equilibrium structure in the excited state. The failure to assign most of the vibrational structure may be due to a number of factors. The spectrum may be complicated by the Renner± Teller e€ect, by contributions from more than one excited electronic state, and by the occurrence of stretch-bend coupling between the two low-frequency vibrational modes. Instead of exhibiting simple behaviour, it therefore seems likely that the ~ state region is subject to a variety of perturbaC

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tions which will be a signi®cant challenge to understand. We hope that the present low resolution study will provide the stimulus for further work on this interesting problem. A high-resolution inves~ X ~ spectrum would be particutigation of the C± larly welcome. Acknowledgements The authors would like to thank the UK Engineering and Physical Sciences Research Council for the award of a studentship to GMG. References [1] L. Pasternack, P.J. Dagdigian, J. Chem. Phys. 65 (1976) 1320. [2] C.J. Whitham, B. Soep, J.-P. Visticot, A. Keller, J. Chem. Phys. 93 (1990) 991. [3] T.C. Steimle, D.A. Fletcher, K.Y. Jung, C.T. Scurlock, J. Chem. Phys. 97 (1992) 2909. [4] C.T. Scurlock, D.A. Fletcher, T.C. Steimle, J. Chem. Phys. 101 (1994) 7255. [5] M. Douay, P.F. Bernath, Chem. Phys. Lett. 174 (1990) 230. [6] G.M. Greetham, A.M. Ellis, J. Chem. Phys. 113 (2000) 8945. [7] M.S. Beardah, A.M. Ellis, J. Chem. Phys. 110 (1999) 11244. [8] G. Greetham, A.M. Ellis, J. Mol. Spectrosc., accepted. [9] P.F. Bernath, Science 254 (1991) 665. [10] D.H. Hildenbrand, J. Chem. Phys. 48 (1968) 3657. [11] E. Murad, J. Chem. Phys. 75 (1981) 4080. [12] D.V. Lanzisera, L. Andrews, J. Phys. Chem. 101 (1997) 9666. [13] G. Herzberg, Electronic spectra of polyatomic molecules, Van Nostrand Reinhold, New York, 1966. [14] P.S.H. Bolman, J.M. Brown, Chem. Phys. Lett. 21 (1973) 213. [15] C.W. Bauschlischer, S.R. Langho€, H. Partridge, Chem. Phys. Lett. 115 (1985) 124. [16] S. Nanbu, S. Minamino, M. Aoyagi, J. Chem. Phys. 106 (1997) 8073. [17] R.R. Wright, T.A. Miller, J. Mol. Spectrosc. 194 (1999) 219. [18] T.C. Steimle, S. Sato, S. Takano, Astrophys. J. 410 (1993) L49. [19] C.T. Scurlock, T.C. Steimle, R.D. Suenram, F.J. Lovas, J. Chem. Phys. 100 (1994) 3497. [20] M.A. Anderson, T.C. Steimle, L.M. Ziurys, Astrophys. J. 429 (1994) L41. [21] M.A. Anderson, L.M. Ziurys, Chem. Phys. Lett. 231 (1994) 164.