Two-dimensional REMPI of CF3Br: Rydberg states and photofragmentation channels

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Abstract

Mass spectra were recorded for (2 + n) REMPI of CF3Br as a function of resonance excitation energy in the 71 320–84 600 cm−1 region to obtain two-dimensional REMPI data. CF3+, iBr+ (i = 79,81) and C+ ions were detected. Laser power dependence experiments were performed for ion signals. CF3+ signals due to resonance transitions from the ground state CF3Br to number of Rydberg states and various vibrational states followed by CF3+ and Br ion-pair formation via intersystem crossings, were identified. Bromine atom production by predissociation channels via two-photon excitation to the Rydberg states is proposed, based on detecting bromine atom (2 + 1) REMPI.

Highlights

► (2 + n) REMPI of CF3Br are studied for the 71 320–84 600 cm−1 excitation region. ► REMPI of CF3Br reveal CF3+, iBr+ and C+ ion signals. ► Resonance excitations to characterised Rydberg states are observed. ► Excitations to Rydberg states are followed by ion-pair formations. ► Predissociation of Rydberg states is observed.

Introduction

CF3Br and its related counterparts CF3Cl and CF3I are all important industrial compounds with wide ranging applications. The freon CF3Br (Halon 1301) has been used in aircrafts for fuel inerting and as a fire suppressor whereas CF3Cl (CFC-13) has been used as a refrigerant. The production and use of these two compounds has been banned under the terms of the Montreal Protocol since 2000 [1], due to their significant ozone depletion and global warming properties. More recently CF3I, which is considered less environmental unfriendly, has been considered as a possible replacement compound for CF3Br and CF3Cl. All in all the possible uses or discarding of these compounds rely on their photochemical and photoabsorption properties.

Rather limited detailed information are available on the photoabsorption and photochemistry of CF3Br as well as of CF3Cl and CF3I. Although the vacuum UV spectrum of CF3Br was reported as early as 1973 [2] only recently reliable absorption cross sections at energies above the lowest lying electronic transitions have appeared [3]. The UV absorption spectroscopy of these compounds is similar. Weak continua due to n  σ C–X (X = Cl, Br, I) transitions (A bands) appear at low energies. More intense structures due to σ  σ C–X transitions and transitions from lone pair orbitals with dominant X character (p to sp3 character) to ns and np Rydberg states appear at higher energies [3], [4]. Rydberg states identified, are either believed to belong to series converging to the two spin–orbit components of the ground state ions, X2E3/2 and X2E1/2, or the first excited ionic state A2E1/2 [3], [5]. Spectra due to transitions to ns Rydberg states are generally found to be more intense than those due to transitions to np states as one might expect from the basic selection rule Δl = ±1.

In terms of photochemistry studies main emphasise has been on the effect of absorption in the low energy A band corresponding to transitions to several repulsive states followed by dissociation to form CF3 + Br(2P3/2)/Br(2P1/2) [6], [7], [8]. The same products are believed to be the main result of photodissociation via higher energy states [4], [9]. Fluorescence studies of CF3Cl and CF3Br reveal emissions from CF3 for excitations above the energy thresholds for CF3 + X; X = Cl, Br formations [4], [10], [11] with maximum quantum yields of about 20% and 7% at 112.5 nm (CF3Cl) and 125.0 nm (CF3Br), respectively. Judging from negative photoion spectroscopy studies of CF3X; X = Cl, Br and I insignificant ion-pairs, CF3+ + X are formed via excitations to the high energy Rydberg state region despite being energetically allowed [9]. This is surprising and contradicts with observations for related compounds such as methyl halides [12], [13], hydrogen halides [14], [15], [16], [17], halogens [18], [19] and interhalogens [20], [21] where Rydberg doorway states in ion-pair formations have been evidenced.

Whereas resonance enhanced multiphoton ionization (REMPI) studies have been performed for CF3Cl [11] and CF3I [22], [23] to our best knowledge no analogous studies have been reported for CF3Br. 4p and 6p Rydberg states are found to be excited by two-photon resonance excitations of CF3Cl and CF3I, respectively. Spectra due to transitions to Rydberg states with total electronic angular momentum quantum numbers Ω = 0,1 and 2 in the series converging to both ion states, X2E3/2 and X2E1/2, have been identified for CF3I [22]. The fragment ions CF3+ and CF+ and (2 + 1) REMPI of the Cl atoms are detected in REMPI of CF3Cl, whereas no parent ions have been seen [11]. The CF3+ ions are believed to be formed by ionization of CF3 after dissociation of the resonance excited Rydberg states and the strong Cl atom resonances are mainly attributed to dissociation channels which form the species CF3 + Cl(2P3/2)/Cl(2P1/2). CF3+ and CF3I+ ions are detected in REMPI of CF3I. It has been argued that structure distortions observed in Rydberg states of CF3I could possibly be due to interactions with ion-pair states [22].

The Rydberg and ion-pair state structures of the unfluorinated counterparts CH3X; X = Cl, Br and I have been studied more extensively than the corresponding fluorinated compounds [24] and limited comparison of spectra structures is available. Considerable differences, however, have been observed in certain Rydberg spectra structures for CF3I and CH3I [22].

In this Letter we present (2 + n) REMPI studies of CF3Br. Two-photon resonance excitations to Rydberg states are observed and characterized. Ion-pair formations and predissociation channels are identified.

Section snippets

Experimental

Two dimensional (2D) REMPI data for jet cooled CF3Br gas were recorded. Ions were directed into a time-of-flight (TOF) tube and detected by a micro-channel plate (MCP) detector to record the ion yield as a function of mass and laser radiation wavenumber. The apparatus used is similar to that described elsewhere [25], [26], [27]. Tunable excitation radiation in the 562–475 nm wavelength region was generated by Excimer laser-pumped dye laser systems, using a Lambda Physik COMPex 205 Excimer laser

Results and analysis

2D-REMPI data recorded for the two-photon excitation region 71 320–84 600 cm−1 revealed ion signals for CF3+, iBr+ (i = 79,81) and C+ only. The CF3+ 1D-REMPI showed weak broad features ranging from about 74 300 cm−1 to 84 600 cm−1 (see Figure 1). The atom ion signals consisted of iBr(2P3/2)/iBr(2P1/2) (Figure 2) and C(3PJ)/C(1D) atom (2 + 1) REMPI’s only. The iBr atom (2 + 1) REMPI signals observed, were very weak in the lowest wavenumber region (70 987–72 012 cm−1; Br∗∗([3P2]c 5p) ←← Br(4p5; 2P1/2)

Conclusions

(2 + n) REMPI of CF3Br reveals weak CF3+, medium to strong iBr+ (i = 79.81) and very weak C+ ion signals, only, in the two-photon excitation region 71 320–84 600 cm−1. The CF3+ signal shows molecular structures whereas only (2 + 1) atom REMPI signals for iBr+ and C+ are observed. The CF3+ signals are associated with initial two-photon excitation to p and d Rydberg states followed by crossing to ion-pair states and formation of CF3+ and Br ion pairs in agreement with general observations for large

Acknowledgements

The financial support of the University Research Fund, University of Iceland, the Icelandic Science Foundation as well as the Norwegian Research Council is gratefully acknowledged.

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