Rotationally Resolved Vacuum Ultraviolet Resonance-Enhanced

Article pubs.acs.org/JPCA

Rotationally Resolved Vacuum Ultraviolet Resonance-Enhanced Multiphoton Ionization (VUV REMPI) of Acetylene via the G̃ Rydberg State Alice F. Schmidt-May,†,‡ Monika Grütter,†,‡ Jannis Neugebohren,†,‡ T. N. Kitsopoulos,†,‡,§,∥ Alec M. Wodtke,†,‡ and Dan J. Harding*,†,‡ †

Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstraße 6, 37077 Göttingen, Germany Max-Planck-Institut für biophysikalische Chemie, Am Fassberg 11, 37077 Göttingen, Germany § Department of Chemistry, University of Crete, 71003 Heraklion, Greece ∥ Institute of Electronic Structure and Laser, Foundation for Research and TechnologyHellas, 71003 Heraklion, Greece ‡

ABSTRACT: We present a 1 + 1′ resonance-enhanced multiphoton ionization (REMPI) scheme for acetylene via the linear G̃ 4sσ 1Πu Rydberg state, offering partial rotational resolution and the possibility to detect excitation in both the cis- and transbending modes. The resonant transition to the G̃ state is driven by a vacuum ultraviolet (VUV) photon, generated by resonant fourwave mixing (FWM) in krypton. Ionization from the short-lived G̃ state then occurs quickly, driven by the high intensity of the residual light from the FWM process. We have observed nine bands in the region between 79 200 cm−1 and 80 500 cm−1 in C2H2 and C2D2. We compare our results with published spectra in this region and suggest alternative assignments for some of the Renner−Teller split bands. Similar REMPI schemes should be applicable to other small molecules with picosecond lifetime Rydberg states.

■

INTRODUCTION

There have been several spectroscopic investigations of acetylene and its isotopologues in the region around 125 nm or 80 000 cm−1 that are particularly important for our work.4−10 Most of the electronic excited states in this region are Rydberg states, whose energy can be predicted according to the Rydberg formula for the appropriate series, making their assignment relatively straightforward. Herzberg first identified the symmetry of the G̃ 1Πu state and the presence of Renner−Teller splitting in the vibrational hot bands of the G̃ state based on the observation of three bands for C2H2 and five for C2D2.4 Herman and co-workers have used absorption spectroscopy to investigate the Renner−Teller interactions in the G̃ and H̃ states5 of acetylene, where they made a complete assignment of the bands observed in the range 120.5−125.5 nm. They later extended this work to other Rydberg states6 where they noted that many of the bands are diffuse and/or overlapping. Löffler et al. were primarily interested in photodissociation dynamics, but have measured spectra of C2H2 and C2D2 using action spectroscopy, by ionization of H atoms produced

Sensitive, quantum-state resolved detection methods are a vital component of many chemical and reaction dynamics studies, providing information about the transfer of energy and the potentials governing collisions or reactions. Recent examples, showing what can be achieved with favorable detection schemes, include the observation of diffraction oscillations in inelastic molecular scattering,1 orientation effects in molecule− surface scattering,2 and steric control of reactions in crossedbeam scattering.3 Acetylene is one of the smallest polyatomic molecules and has been studied in great depth for many years. Despite its small size and simple geometry, changes away from the linear D∞h symmetry of the ground state during bending vibrations or in the low-lying electronic excited states complicate acetylene’s spectroscopy. In the linear states of acetylene, ν4 is the trans-bend and ν5 is the cis-bending mode. The vibrational state of a molecule is then described by the number of quanta in each mode (ν1ν2ν3ν4ν5); i.e., (00000) is the vibrational ground state and (00010) has one quantum of excitation in ν4. Transitions are then labeled by the number of quanta in excited vibrational modes in the ground and excited states, 401 being a transition between the (00010) vibrationally excited ground state and the vibrationless (00000) electronic excited state. © XXXX American Chemical Society

Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: March 9, 2016 Revised: April 13, 2016

A

DOI: 10.1021/acs.jpca.6b02477 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

the earlier assignments of the spectrum and we therefore attempt to provide further evidence to help determine the states involved. This is of particular interest for the linear Rydberg states of acetylene, as they have become the prototype systems for the development of the theory of the Renner− Teller interaction in four-atom molecules.13,14 Parker and co-workers have recently reported a VUV 1 + 1′ REMPI scheme for CO via the A 1Π state.15,16 The experimental set up and ionization process is similar to that reported here; however, the intermediate A 1Π state is not predissociated.17

following dissociation of Rydberg states. These spectra were measured in a supersonic molecular beam and have rotational temperatures of 4 K. The low rotational temperatures allowed the natural line widths and predissociation lifetimes to be determined for many states. The lifetime of the G̃ state was found to be greater than 2 ps, while they determined a lifetime of 52 fs for the H̃ state.8 Gauyacq and co-workers have measured spectra of acetylene Rydberg states using several different methods: Fluorescence from the C2H fragment formed following VUV photodissociation9 was used to measure spectra of the origin bands of the G̃ and H̃ states. Though the G̃ state origin band shows (instrument limited) partial rotational resolution, the H̃ origin band is broad and appears to be made up of two features. The authors suggest that the width of this band (and the H̃ −X̃ 210 band) is evidence for short (75 fs) lifetimes in the H̃ state. They also determined a slightly longer lifetime, of >10 ps, for the G̃ state compared to Löffler et al. This group extended their work using VUV absorption spectroscopy and 3 + 1 REMPI and stated that they were unable to observe the H̃ state in the 3 + 1 REMPI measurements because of its short lifetime.10 The short lifetimes of the H̃ state reported more recently8,9 appear to be in contradiction to the assignment of Herman and co-workers.5 They assigned two rotationally resolved bands (at 80 233.3 cm−1 and 80 304.7 cm−1 in C2D2) to components of the H̃ state 411 triad, though they reported that the H̃ origin band was not rotationally resolved but that, under some conditions, a double headed structure could be seen. We also note that Ashfold et al. found a long-lived Rydberg state by 3 + 1 REMPI which they assigned to the F̃ ′ 1Φu state.7 With T0 = 79 976 ± 2 cm−1 in C2D2, the F̃ ′ state lies rather close to the G̃ state and may have one-photon allowed hot bands in the same region. However, apart from the origin band, only the 210 band in C2D2 was observed in the 3 + 1 REMPI spectra and the (01000) level of the F̃ ′ state was determined to have a predissociation lifetime of 1 ps.7,10 The symmetry of the intermediate excited state in the REMPI process determines which vibrational (for acetylene, strictly (ro-)vibronic) states of the molecule can be detected. For example, the Ã state of acetylene has a trans-bent geometry making detection of cis−bending excitation via Ã −X̃ transitions Franck−Condon inactive. The G̃ state has the advantage of being linear, meaning that transitions from the cis- and transbending hot-bands are not a priori inactive. Acetylene is also an important feed stock in industrial processes, hence the interest in understanding its interactions with surfaces. There have been a only a small number of molecular beam surface scattering experiments reported in the literature,11,12 presumably limited in part by the difficulties involved in quantum-state-specific detection of acetylene under ultrahigh vacum conditions. Miller and co-workers used high resolution IR spectroscopy with bolometric detection to investigate the scattering of acetylene from LiF(111).11 Golibrzuch et al. used 1 + 2 REMPI through the Ã state to probe the rotational product state distribution and excitation of the ν4 trans-bending mode of C2H2 scattered from Au(111).12 Due to the challenges involved in these approaches, we aimed to develop an improved detection scheme to facilitate scattering experiments, and here, we report a new 1 + 1′ REMPI scheme for acetylene based on excitation to the G̃ Rydberg state with vacuum ultraviolet (VUV) light. The main goal of our work was the experimental implementation of this new scheme. However, during the course of our work we have found inconsistencies in

■

EXPERIMENTAL The experiments are performed using a molecular beam apparatus which has been described in detail previously.18 Here,

Figure 1. VUV 1 + 1′ REMPI spectra taken with the tunable red laser at low power (∼3 mJ/pulse). An increase in signal and spectral broadening were observed as the power of the additional 532 nm green ionization laser was increased.

for most of the measurements, we use a time-of-flight configuration with Wiley−McLaren extraction fields19 and read the ion signal from the back of the MCP instead of using the slice imaging configuration and position sensitive detection. The heated pulsed nozzle18,20 was used to increase the population of vibrationally excited states, helping to identify hot bands. In contrast to an effusive beam or a gas cell where the sample will have a Boltzmann population, the beam produced in this way still undergoes significant rotational cooling, leading to different effective temperatures for rotation and vibration. Due to inefficient cooling of vibrations via collisions, the vibrational temperatures are expected to be significantly higher than the rotational temperature. The valve was backed with a mixture of 15% C2H2 or C2D2 in He at a pressure of 2 bar. We have added a cell for VUV generation; this is 250 mm long and has a 50 mm focal length LiF lens between the krypton-filled region and the high vacuum detection region. Tunable VUV is generated by resonant four-wave difference frequency mixing21,22 using a two-photon resonance in krypton at 212.5 nm (∼3 mJ/pulse from a tripled dye laser) and a tunable dye laser (∼30 mJ/pulse) between 670 nm and 735 nm, hereafter referred to as the red beam. Both dye lasers are pumped by the same doubled Nd:YAG laser. On the basis of the specifications of the dye lasers, we expect the VUV line width to be on the order of 0.7 cm−1. The two beams are focused into the cell filled with 20 mbar of Kr using a 200 mm lens. A telescope in the red beam is used to adjust the overlap of the focal points. The focus of the VUV and residual beams in the extraction region of the mass spectrometer can be adjusted by changing the focusing conditions into the cell. We found the B


Article


Table 1. Values for the Harmonic, l-Anharmonic, Rotational, Λ-Splitting qe, and the Centrifugal Correction D, Constants for the Electronic Ground State of 12C2H2 and 12C2D2 As Listed by Herman et al.25 Except for the Values Where the References Are Given

optimum signal to be obtained with both the VUV and the residual red light only loosely focused in the molecular beam. To ensure that the signal we observe is due to the VUV, we have also performed scans without krypton in the cell. Under these conditions we do not observe any ions, showing that the VUV is necessary for ionization. We have corrected for changes in the VUV power over the scan range by measuring onephoton ionization spectra for acetone and NO as a function of VUV wavelength. REMPI spectra are obtained by monitoring the ion signal in the appropriate m/z channel (26 for C2H2 and 28 for C2D2) The mass resolution is sufficient to separate the parent ion from fragments due to the loss of an H(D) atom. Ionization from the G̃ state could, energetically, be driven by any of the three colors (VUV, UV, or red) present in the interaction region. In order to test which is responsible, we have measured a photoelectron spectrum (PES) using velocity map imaging. The PES shows that the much higher flux of residual red photons leads to efficient ionization via the VUV + red channel, and we see no evidence for the other ionization routes. Thus, the total ionization energy is always the same (2 × UV energy), meaning there are no effects in the spectrum due to potential autoionizing resonances. We have tested the possibility of using an additional frequency doubled Nd:YAG (532 nm) laser, called the green laser hereafter, to enhance the ionization signal, and although this works, the enhancement in this case is relatively small (approximately a factor of 2) but the spectral resolution is significantly reduced. In cases where the residual red laser does not have sufficient photon energy to ionize the intermediate excited state this could, however, be a useful addition. The highest resolution spectra we have obtained have a line width of 1.5 cm−1, around twice the line width we expect for the VUV. Saturation broadening of the G̃ −X̃ transition with VUV seems extremely unlikely under the present focusing conditions. We have tested the effects of the ionization laser by running the red laser at low power (∼3 mJ/pulse); this reduces the VUV flux but allows us to control the power driving the ionization step using the additional Nd:YAG laser. Figure 1 shows spectra measured in this way and clearly shows that the green ionization laser alone can cause broadening of the spectral line width. This in turn suggests that the strong electric field of the focused laser causes a Stark shift of the Rydberg states.23 The fact that we observe a broadening rather than a shift of energy shows that we observe an ac Stark shift. Saturation effects in the VUV generation may also cause broadening of the VUV beyond that expected based on the line widths of the driving lasers.8 Wavelength calibration of our spectra was made by comparison to the reported positions of the G̃ origin band and VUV 1 + 1′ REMPI spectra on the 1 0 5d⎡⎣ 2 ⎤⎦ state of xenon.24 The positions of the other bands then 1 agree to within 1 cm−1 to the positions reported by Colin et al.5 On the basis of the observed pressure rise in the vacuum chamber and simple modeling of the molecular beam, we estimate the lower detection limit for ground state acetylene via the origin band to be of the order of 108 molecules/cm3 per rotational state. The sensitivity is sufficient to allow the detection of the previously unobserved 401 hot band (vide infra).

C2H2, cm−1

12

ω1 ω2 ω3 ω4 ω5 g44 g55 g45 B0 B′ D0 D00010 qv,00010 qv,00001 qe′ D′

3397.12 1981.80 3316.86 608.73 729.08 0.497 3.508 6.828 1.17664632 1.1825444 1.61 × 10−6 [28] 1.64 × 10−6 [27] 5.2486 × 10−3 [30] 4.66044 × 10−3 [30] 0.80 × 10−2 [5] 1.5 × 10−6

12

C2D2, cm−1

2717.22 1768.07 2455.11 509.237901 537.9979 0.4512 2.2670 3.18274 0.84787420 0.850241 0.80 × 10−6 [28] 0.81 × 10−6 [27] 3.24200 × 10−3 [31] 3.27567 × 10−3 [31] (0.8 ± 0.2) × 10−2 0.66 × 10−6 [5]

symmetry of the different possible vibronic states involved in the transition. Key elements for the assignments in this region are the band profile and the observation, or lack thereof, of intensity alternations of adjacent rotational levels due to nuclear spin statistics, all governed by the symmetries of the states involved in the transition. The resonant transition in the REMPI scheme we employ is a one photon absorption. In our evaluation, we treat the spectra identically to one photon absorption spectra, assuming that the ionization probability is identical for each rotational state within a band. The line positions were calculated based on established formulas for vibrational25 and rotational26 energies and convoluted with a Lorentzian line shape. The Lorentzian line width was adjusted to match each band in the experimental spectrum. In the description of the state energies, we have limited ourselves to first order anharmonicities in the description of the vibrational energy and the first centrifugal correction of the rotational energy. The constants for the electronic ground state were taken from ref 25 except the centrifugal distortion constants which were taken from Ghersetti et al.27 and Palmer et al.28 For the intermediate G̃ 1 Πu state, we used the constants from Colin et al.5 and adjusted them to the experimental spectrum when needed (see results). Acetylene and dideuterated acetylene are linear in their ground state and have five vibrational modes, three nondegenerate stretches, and two doubly degenerate bending modes. The two doubly degenerate bending modes are of much lower energy compared to the stretches. Therefore, at room temperature, the vibrational ground state and one quantum excitation in either of the two bending modes contribute to more than 94% of the entire population. All other vibrational states have a population of 1% or less and were not considered in our analysis. The complexity of the spectroscopy of acetylene stems from the change of symmetry from the linear ground state, caused by electronic excitations or vibrational motion. Bending of the otherwise linear molecule causes a vibrational angular momentum l that couples to the other angular momenta. It

■

THEORY In order to help assign our experimental spectra, we have calculated line positions and simulated spectra for comparison with the measured spectra. The simulations are based on the C


Article


the agreement between calculated and measured spectra. In the case of Δ−Π transitions we did not include the l-splitting, as we found it to be much smaller than our experimental resolution. The 1Σg electronic ground state of the acetylene isotopologues has no electronic angular momentum, but the presence of orbital angular momentum in the electronically excited state leads to Renner−Teller splitting of the electronic state into vibronic states of different energy as soon as there is vibrational angular momentum.13,29,32 One quantum of excitation in the trans bending mode, which is of πg symmetry, in the electronic excited 1Πu state creates four states with three different energies: 1Σ−u , 1Σ+u , and 1Δ±. In the same electronic state, one quantum of excitation in the cis bending mode with πu symmetry creates 1Σ−g , 1Σ+g , and 1Δ± states. It is extremely difficult to predict the energies of these vibronic states. Perić et al. performed ab initio calculations, but their results do not agree well with the observed positions of the bands.14 Solutions to the Renner equations describing the vibrational energy in molecules of the form X2Y2 were derived by Petelin and Kiselev in 1972 using perturbation theory up to second order.13 By application of their formulas to the G̃ 1Πu state with one quantum excitation in the bending mode i, the vibronic states have the energy G Σ = ωi ± ωi ϵi

(1)

and GΔ = ωi −

1 ωi ϵi 2 2

(2)

relative to the vibrational ground state with ωi being the vibrational constant and ϵi the Renner−Teller parameter of the bend i. It is not known a priori if the Σ+ state is of higher energy than the Σ− state, i.e., if the (+) sign in eq 1 belongs to the Σ+ or Σ− state.29 The ab intio calculations of Perić et al. however suggest the Σ− component to be of lower energy.14 The splitting between the Σ components is |G Σ+,1,0 − G Σ−,1,0| = 2ωi ϵi

(3)

For typical small values of ϵ the Δ state lies between the two Σ components in energy. The spectrum was simulated by calculating the intensity, I, of each rovibronic line using ⎡ − E ″ (J ″ ) ⎤ I = C exp⎢ rot ⎥ g (J ″) HL△Λ,branch(Λ,J ) ⎣ kBT ⎦ nspin

Figure 2. Schematic display of the intensity alterations expected for P, Q, and R branches of the Renner−Teller split 411 and 511 bands and the 401 band of C2H2. The rotational quantum number J is shown to the right of each level. The nuclear spin statistical weight of the levels is shown using solid lines for the majority states and dashed lines for the minority states. The intensity alternations in the spectrum are shown using thick and thin lines for the strong and weak lines, respectively (after Colin et al.5).

(4)

Each band has its own arbitrary scaling factor C. The rotational state distribution was described using the Boltzmann factor, ⎡ −E ″ (J ″) ⎤ exp⎢⎣ krotT ⎥⎦, with the rotational energy E″rot(J″) of the lower B state and the Boltzmann constant kB. The apparent rotational temperature T was adjusted to each band individually. gnspin(J″) is the nuclear spin statistical weight for a given initial state. The Hönl−London factors HL△Λ,branch(Λ,J) were taken from ref 26, where they are tabulated for diatomic molecules as a function of the electronic angular momentum Λ. They are also applicable for linear polyatomic molecules, and for our simulations Λ was replaced with the vibronic angular momentum K. The molecular constants used in our simulations are shown in Table 1. The dependence of the relative intensity of the different rotational branches on the state symmetries causes different band profiles for the different transitions, which make the band

leads to a splitting in rotational levels, the so-called l-doubling, analogous to the Λ-doubling in the interaction of the rotational and electronic angular momentum Λ.26,29 In the calculation of line positions, we included these splittings for the first excited trans- and cis-bends in the electronic ground state using the constants of refs 30 and 31. The coupling between the vibrational and the electronic angular momentum creates the vibronic angular momentum K = |±Λ ± l|, and the vibrational and electronic states become inseparable vibronic states. We initially used the Λ-doubling constants for the G̃ 1Πu state of Colin et al.5 but had to adjust it in the case of C2D2 to improve D


Article


Figure 3. Experimental overview spectra of C2H2 and C2D2 measured with the nozzle at room temperature and hot. The numbering of the bands corresponds to that used in the text. Bands 5 and 8 are were not observed for C2H2.

profiles useful for assigning the transition. A second possibility to support assignments is introduced by the number of suitable nuclear spin wave functions gnspin(J″) for a given rovibronic state. The ratio of symmetric to antisymmetric nuclear wave functions is 1:3 in 12C2H2 and 6:3 in 12C2D2. As the parity of the rotational wave function alternates from one rotational state to another, every second rotational state has a higher statistical weight due to a greater number of suitable nuclear wave functions to fullfill the fermionic or bosonic criterion for the symmetry of the total wave function. Due to the parity of the vibronic states, the rotational states with odd numbered total angular momentum J have a higher statistical weight than those with even numbers in Σ+g and Σ−u and have a lower weight in Σ+u and Σ−g . Figure 2 shows schematically the intensity alterations in the different types of transitions that we consider for bendexcited acetylene. A vibronic Π, Δ, ... state always consists of two states that are in a first approximation degenerate but usually split into two by l-doubling or Λ-doubling. These two states have different parities and therefore show opposite intensity alternation in a spectrum. If they are unresolved, this leads to an apparent ratio of 1:1 for odd and even numbered J states and therefore no visible intensity alternations in the spectrum.

Figure 4. Comparison of measured and simulated spectra for band 6, the G̃ 000 origin bands of C2H2 (a) and C2D2 (b) with the nozzle at room temperature. The widths of the Lorentzian line shape convolutions and the rotational temperatures are shown, and the rotational levels (J″) of the ground state are labeled.

■

RESULTS AND DISCUSSION Figure 3 shows overview spectra for C2H2 and C2D2 taken with the nozzle at 22 °C and 170 °C, called cold and hot spectra hereafter. In addition to the strong origin bands, labeled 6, there are a number of other weak bands visible. The intensity of these bands relative to the origin band increases with increasing nozzle temperature, strongly suggesting that they are vibrational hot bands and consistent with earlier assignments. We observe two extra bands, labeled 5 and 8, in the spectrum of C2D2 compared to C2H2. The bands we observe appear to correspond to those observed by Löffler et al. in this region,8 with the exception of the broad band centered at 80 450 cm−1

assigned to the short-lived H̃ state. We have measured spectra, not shown, covering the region of the H̃ origin band but were unable to observe any ion signal. In the following, we will concentrate on the spectra of C2D2. The symmetry assignments that we make are consistent with our results for C2H2. Band 6, shown in Figure 4, is the origin 000 band of the G̃ state and is well reproduced by the simulated rotational structure for a Πu−Σ+g transition for both isotopologues. At high nozzle temperature we observed the growth of a broad feature under the P branch of the 000 band, suggesting that at least one hot band also lies in this region. E


Article


Figure 5. Comparison of measured and simulated spectra of band 1 for (a) C2H2 and (b) C2D2. The labels for the rotational levels (J″) of the P branch neglect the l-doubling in the (000010) state which is included in the simulation.

Figure 7. Spectra of bands 5 and 7. (a) Band 5 is only observed for C2D2, and comparisons are shown for Σ−Π (red) and Δ−Π (green) transitions. For band 7, spectra for both isotopologues (b, c) are shown together with simulations for Σ−Π transitions.

Figure 8. Spectra of bands 8 and 9 for C2D2. (a) The experimental spectrum of band 8 is shown with the simulated spectrum for a Σ−Π transition. (b) Comparison of the experimental spectrum of band 9 with simulations for both Σ−Π and Δ−Π transitions. A better match to the experiment is seen for the Δ−Π simulation. Figure 6. Comparison of measured spectra and simulations for band 2 in C2D2 with the nozzle at room temperature (a) and hot (b). Simulations are shown for Σ−Π (red) and Δ−Π (blue) transitions.

Band 2 was suggested by Colin et al.5 to be formed by two overlapping bands including one of the Σ components of the 411 triad of the G̃ state. Our results, shown in Figure 6, are consistent with the presence of two bands, seen by the extra intensity under the P and Q branches, but we suggest a Δ−Π transition is more likely, given that there are no intensity alterations between odd and even rotational lines. The effect of the second band is relatively small at low temperature but is presumably the cause of the rather poor agreement between the Δ−Π simulation and the measured spectra. At high nozzle temperature, while the R branch remains clearly visible, the P and Q branches become obscured. Bands 3 and 4 were too weak and poorly resolved to allow further analysis. Band 5, centered around 80 050 cm−1, was assigned by Colin et al.5 as the Δu−Πg component of the 411 triad of the G̃ state. Figure 7a shows our measured spectrum and comparison to

Band 1 is shown in Figure 5; it is weak and has not been reported previously. It lies to the red of the G̃ state origin band by 613 cm−1 in C2H2 and 512 cm−1 in C2D2, consistent with the energy difference between the (00000) and (00010) vibrational levels in the X̃ 1Σg ground state; therefore we assign it to the 401 hot band of the G̃ state. This band is straightforward to assign as there is no Renner−Teller splitting because the excited state has no vibrational excitation, and comparison of the experimental and simulated spectra shows a good agreement for a Πu−Πg transition. The simulated spectra use the previously published molecular constants,5,25,27,28,30,31 shown in Table 1, except for qe of C2D2 for which we found a better agreement using a value of (0.8 ± 0.2) × 10−2 cm−1 instead of (4 ± 2) × 10−2 cm−1. F


Article


Table 2. Assignments, State Symmetries, and Values of the Band Origin T0 and Rotational Constants in the Electronic Excited State B′ Used in the Simulationsa T0, cm−1 band no. 1 2 3 4 5 6 7 8 9

assignment 401

Πu−Πg (Δ−Π)

511 000 511 411

Σ−u −Πg Πu−Σ+g Σ+u −Πg Σ−u −Πg (Δ−Π)

B′, cm−1

C2H2

C2 D 2

C2H2

C2D2

assignment from ref 5

79498.1 79790.1 79861 79886

79637.2 79865.4 79905 79938 80004 80149.3 80180.3 80232.6 80307.5

1.102 1.12

0.797 0.80

411, G̃ Σu−Πg

0.81 0.797 0.806 0.800 0.805

411, 000, 411, 411, 411,

80111.0 80154.5 80305

1.102 1.1 1.13

G̃ Δu−Πg G̃ Πu−Σ+g G̃ Σu−Πg H̃ Σ−u −Πg H̃ Δu−Πg

Fitting errors are about ±1 on the last given significant figure, and the entire uncertainty of the band position is estimated to be about 1 cm−1 due to the calibration method. The parentheses indicate that the symmetry was used in the simulation but that the agreement with the spectra was too poor to allow an assignment to this symmetry. a

longer lifetimes than the (00000) and (01000) levels. Instead, it seems more likely that these bands belong to the G̃ state. Band 9, shown in Figure 8b, was also assigned by Colin et al. to the H̃ state, specifically to the Δ−Π component of the 411 triad.5 Simulation of a Δ−Π transition did not match the experimental spectrum well but was in much better agreement than a Σ−Π transition. Table 2 shows the transition energies and symmetries that we assign for the different bands, together with those of Colin et al.5 From the symmetries that we assign to the bands we suggest that bands 5 and 7 belong to one triad, but without resolving the l-doubling in the lower state it is not possible to determine if it is the 411 or the 511. Colin et al. were, however, able to assign band 8 to a Σ−u −Πg in this way5 (albeit in the H̃ state), suggesting that this band is in fact one of the 411 components and therefore that bands 5 and 7 belong to the 511 triad. Calculating the expected positions of the Δ components based on this assumption places the 511 Δ component under the 000 origin band at 80 084 cm−1. Assuming the coupling is of similar strength in the 411 triad, its Δ component would be found at 80 132 cm−1. This could provide one explanation why we are unable to observe the Δ components and an explanation for the hot band intensity we observe under the origin band. The harmonic frequencies and Renner parameters, shown in Table 3, would, in this case, be significantly different in the G̃ state and the X̃ 2Πu ground state of the cation,33−35 as previously noted by Tang et al.36 Such differences seem reasonable given that the G̃ state is a relatively low Rydberg state where the electron may still have significant interaction with the ionic core. The observed Δ−Π bands could be explained by transitions belonging the 411 and 511 bands of the F̃ ′ 1Φu state. The F̃ ′−X̃ origin band is forbidden for one-photon transitions, but Renner−Teller splitting in the upper state gives components with Δ and Γ symmetry. Transitions to the Δ components from the Π vibrationally excited ground state are symmetry allowed for one-photon transitions and would lead to Δu−Πg (for 411) and Δg−Πu (for 511) bands. A second, more speculative, possibility is that the small energy difference and identical symmetry of the 411 and 511 Δ states leads to an interaction which pushes the Δ states apart and that the Δ−Σ components we observe belong to the G̃ −X̃ 511 and 411 triads.

Table 3. Harmonic Vibrational Constants Determined for the Cis Bending Mode and Renner−Teller Parameter in the G̃ State Assuming the Assignment Described in the Texta 12

C2H2 12 C2 D 2

ω5′ , cm−1

ϵ5

646 ± 3 481 ± 2

[0.183 ± 0.002] 0.183 ± 0.002

a

The Renner−Teller parameter in brackets was imposed due to expected invariance between the isotopologues of identical symmetry. The harmonic vibrational frequencies in the ground state were taken from Herman.31

simulated spectra for Δ−Π and Σ−Π transitions. In the simulations, B′ and Trot were fitted independently for each of the possible transitions. The simulation for a Δ−Π transition does not reproduce the measured data, as the ratio of the Q to R branch, governed by the Hönl−London factors, is incorrect. The observed intensity alterations are also inconsistent with a Δ−Π transition. Comparison with the simulation for a Σ−Π transition shows good agreement with the band shape. It is not possible to determine whether the transition is a Σ−u −Πg or Σ+g −Πu, i.e., whether the band belongs to the 411 or 511 triad, without resolving the l-doubling in the lower state, which we are unable to do for the relatively low rotational states in our molecular beam. Band 7 lies close to the R branch of the origin band, leading to overlap with the P branch in particular. However, at low rotational temperatures intensity alterations can be clearly observed and the measured spectra fit well with simulations for a Σ−Π transition, consistent with the assignment of Colin et al. of this band as a Σ component of the 411 triad of the G̃ state.5 Band 8 is shown in Figure 8a. The experimental spectrum is well matched by the simulated spectrum for a Σ−Π transition, but we are unable to resolve the splitting in the lower Π state which would allow the distinction between Σ−u −Πg and Σ+g −Πu transitions. However, Colin et al. wrote about band 8: “In only one case has a unique assignment been obtained from the rotational analysis [...]; the C2D2 band [8][...] was identified as a Σ−u −Πg transition”.5 They assigned it to a component of the 411 triad belonging to the H̃ Rydberg state. Given the short lifetimes subsequently measured for the H̃ state by Löffler et al.8 and Boyé et al.,9,10 the probability of these rotationally resolved bands belonging to the H̃ state and being detectable using our REMPI scheme seems low and would require that the (00010) levels of the H̃ state have much G


Article


■

(11) Francisco, T. W.; Camillone, N., III; Miller, R. E. Rotationally Inelastic Scattering of C2H2 from LiF(100): Translational Energy Dependence. Phys. Rev. Lett. 1996, 77, 1402−1405. (12) Golibrzuch, K.; Baraban, J. H.; Shirhatti, P. R.; Werdecker, J.; Bartels, C.; Wodtke, A. M. Observation of Translation-to-Vibration Excitation in Acetylene Scattering from Au(111): A REMPI Based Approach. Z. Phys. Chem. 2015, 229, 1929−1949. (13) Petelin, A. N.; Kiselev, A. A. The Renner Effect in Four-Atomic Molecules. Int. J. Quantum Chem. 1972, VI, 701−716. (14) Perić, M.; Peyerimhoff, S. D.; Buenker, R. J. Theoretical Study of the U.V. Spectrum of Acetylene. Mol. Phys. 1985, 55, 1129−1145. (15) Song, L.; Groenenboom, G. C.; van der Avoird, A.; Bishwakarma, C. K.; Sarma, G.; Parker, D. H.; Suits, A. G. Inelastic Scattering of CO with He: Polarization Dependent Differential Stateto-State Cross Sections. J. Phys. Chem. A 2015, 119, 12526−12537. (16) Suits, A. G.; Bishwakarma, C. K.; Song, L.; Groenenboom, G. C.; van der Avoird, A.; Parker, D. H. Direct Extraction of Alignment Moments from Inelastic Scattering Images. J. Phys. Chem. A 2015, 119, 5925−5931. (17) Krupenie, P. The Band Spectrum of CO; National Bureau of Standards: Washington, DC, 1996. (18) Harding, D. J.; Neugebohren, J.; Grütter, M.; Schmidt-May, A. F.; Auerbach, D. J.; Kitsopoulos, T. N.; Wodtke, A. M. Single-field Slice-Imaging with a Movable Repeller: Photodissociation of N2O from a Hot Nozzle. J. Chem. Phys. 2014, 141, 054201. (19) Wiley, W. C.; McLaren, I. H. Time-of-Flight Mass Spectrometer with Improved Resolution. Rev. Sci. Instrum. 1955, 26, 1150−1157. (20) Kohn, D. W.; Clauberg, H.; Chen, P. Flash Pyrolysis Nozzle for Generation of Radicals in a Supersonic Jet Expansion. Rev. Sci. Instrum. 1992, 63, 4003. (21) Hilbig, R.; Wallenstein, R. Narrowband Tunable VUV Radiation Generated by Nonresonant Sum- and Difference-frequency Mixing in Xenon and Krypton. Appl. Opt. 1982, 21, 913−917. (22) Marangos, J. P.; Shen, N.; Ma, H.; Hutchinson, M. H. R.; Connerade, J. P. Broadly Tunable Vacuum-Ultraviolet Radiation Source Employing Resonant Enhanced Sum-difference Frequency Mixing in Krypton. J. Opt. Soc. Am. B 1990, 7, 1254−1259. (23) Delone, N. B.; Krainov, V. P. Stark Shift of Atomic Levels in a Laser Field. Laser Phys. 1992, 2, 654−671. (24) Humphreys, C. J.; Paul, E., Jr. Interferometric Wavelength Determinations in the First Spectrum of 136Xe. J. Opt. Soc. Am. 1970, 60, 1302−1310. (25) Herman, M.; Campargue, A.; El Idrissi, M.; Vander Auwera, J. Vibrational Spectroscopic Database on Acetylene, X̃ 1Σg+ (12C2H2, 12 C2D2, and 13C2H2). J. Phys. Chem. Ref. Data 2003, 32, 921. (26) Herzberg, G. Molecular Spectra and Molecular Structure, 2nd ed.; Van Nostrand Reinhold Company: New York, 1950; Vol. II. (27) Ghersetti, S.; Pliva, J.; Rao, K. N. Dideuteroacetylene bands in the 2−2.5 and 5−10 micron regions. J. Mol. Spectrosc. 1971, 38, 53− 69. (28) Palmer, K. F.; Mickelson, M. E.; Rao, K. N. Investigations of Several Infrared Bands of 12C2H2 and Studies of the Effects of Vibrational Rotational Interactions. J. Mol. Spectrosc. 1972, 44, 131− 144. (29) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand Reinhold Company: New York, 1966; Vol III. (30) Kabbadj, Y.; Herman, M.; Di Lonardo, G.; Fusina, L.; Johns, J. W. C. The Bending Energy Levels of C2H2. J. Mol. Spectrosc. 1991, 150, 535−565. (31) Herman, M.; El Idrissi, M. I.; Pisarchik, A.; Campargue, A.; Gaillot, A.-C.; Biennier, L.; Di Lonardo, G.; Fusina, L. The Vibrational Energy Levels in Acetylene. III. 12C2D2. J. Chem. Phys. 1998, 108, 1377−1389. (32) Bunker, P. R. Molecular Symmetry and Spectroscopy; Academic Press: New York, 1979. (33) Jagod, M.; Rösslein, M.; Gabrys, C. M.; Rehfuss, B. D.; Scappini, F.; Crofton, M. W.; Oka, T. Infrared Sectroscopy of Carbo-ions. VI. CH Stretching Vibration of the Acetylene ion C2H2+ and Isotopic Species. J. Chem. Phys. 1992, 97, 7111−7123.

CONCLUSIONS We have presented a new 1 + 1′ REMPI scheme in the VUV for acetylene via the G̃ state. The technique allows ionization through relatively short-lived (tens of picoseconds) intermediate states but not through the shortest lived (few femtoseconds) states, simplifying the observed spectrum compared to absorption or action spectroscopy. Despite this simplification, a complete and unambiguous assignment of the observed bands was not possible, but comparison of simulated spectra based on the state symmetries suggests that some of the earlier assignments were incorrect. Double resonance experiments, for example, pumping the IR-active cis-bend, would significantly help to clarify the assignment. Similar VUV REMPI schemes should be applicable to other molecules with picosecond or longer lifetime Rydberg states.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 551 3912 599. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS M.G. and A.M.W. acknowledge the support of the Alexandervon-Humboldt-Stiftung.

■

REFERENCES

(1) von Zastrow, A.; Onvlee, J.; Vogels, S. N.; Groenenboom, G. C.; van der Avoird, A.; van de Meerakker, S. Y. T. State-Resolved Diffraction Oscillations Imaged for Inelastic Collisions of NO Radicals with He, Ne, and Ar. Nat. Chem. 2014, 6, 216−221. (2) Bartels, N.; Golibrzuch, K.; Bartels, C.; Chen, L.; Auerbach, D. J.; Wodtke, A. M.; Schäfer, T. Observation of Orientation-Dependent Electron Transfer in Molecule-Surface Collisions. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 17738−17743. (3) Wang, F.; Lin, J.; Liu, K. Steric Control of the Reaction of CH Stretch-Excited CHD3 with Chlorine Atom. Science 2011, 331, 900− 903. (4) Herzberg, G. Determination of the Structures of Simple Polyatomic Molecules and Radicals in Electronically Excited States. Discuss. Faraday Soc. 1963, 35, 7. (5) Colin, R.; Herman, M.; Kopp, I. Renner-Teller Interactions in the Acetylene Molecule. Mol. Phys. 1979, 37, 1397. (6) Herman, M.; Colin, R. High Resolution Spectroscopic Study of the Rydberg Series of the Acetylene Isotopic Molecules. Phys. Scr. 1982, 25, 275. (7) Ashfold, M. N. R.; Dixon, R. N.; Prince, J. D.; Tutcher, B. A 1Φu Rydberg state of Acetylene Revealed by Gas Phase Multiphotn Ionization Specroscopy. Mol. Phys. 1985, 56, 1185−1199. (8) Löffler, P.; Lacombe, D.; Ross, A.; Wrede, E.; Schnieder, L.; Welge, K. Photofragment Action Spectroscopy of the Acetylene Molecule in the Vacuum Ultraviolet. Chem. Phys. Lett. 1996, 252, 304−310. (9) Boyé, S.; Campos, A.; Douin, S.; Fellows, C.; Gauyacq, D.; Shafizadeh, N.; Halvick, P.; Boggio-Pasqua, M. Visible Emission from the Vibrationally Hot C2H Radical Following Vacuum-Utraviolet Photolysis of Acetylene: Experiment and Theory. J. Chem. Phys. 2002, 116, 8843−8855. (10) Boyé, S.; Campos, A.; Fillion, J.-H.; Douin, S.; Shafizadeh, N.; Gauyacq, D. Spectroscopy of Acetylene Rydberg States Studied by VUV Absorption and (3+1)-Resonantly Enhanced Multiphoton Ionisation. C. R. Phys. 2004, 5, 239−248. H


Article

The Journal of Physical Chemistry A (34) Pratt, S. T.; Dehmer, P. M.; Dehmer, J. L. Zero-kinetic-energy Photoelectron Spectroscopy from the Ã 1Au State of Acetylene: Renner−Teller Interactions in the Trans-bending Vibration of C2H+2 X̃ 2 Πu. J. Chem. Phys. 1993, 99, 6233−6244. (35) Asvany, O.; Giesen, T.; Redlich, B.; Schlemmer, S. Experimental Determination of the ν5 Cis-Bending Vibrational Frequency and Renner-Teller Structure in Ground State (X̃ 2Πu) C2H2+ Using Laser Induced Reactions. Phys. Rev. Lett. 2005, 94, 073001. (36) Tang, S.-J.; Chou, Y.-C.; Lin, J. J.-M.; Hsu, Y.-C. The Bending Vibrational Levels of the Acetylene Cation: A Case Study of the Renner-Teller Effect in a Molecule with Two Degenerate Bending Vibrations. J. Chem. Phys. 2006, 125, 133201.

I


Rotationally Resolved Vacuum Ultraviolet Resonance-Enhanced

Recommend Documents