Article pubs.acs.org/JPCA
Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques David E. Couch, Henry C. Kapteyn, Margaret M. Murnane, and William K. Peters* JILA and Department of Physics, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: Understanding the ultrafast dynamics of highly excited electronic states of small molecules is critical for a better understanding of atmospheric and astrophysical processes, as well as for designing coherent control strategies for manipulating chemical dynamics. In highly excited states, nonadiabatic coupling, electron−electron interactions, and the high density of states govern dynamics. However, these states are computationally and experimentally challenging to access. Fortunately, new sources of ultrafast vacuum ultraviolet pulses, in combination with electron−ion coincidence spectroscopies, provide new tools to unravel the complex electronic landscape. Here we report time-resolved photoelectron-photoion coincidence experiments using 8 eV pump photons to study the highly excited states of acetone. We uncover for the first time direct evidence that the resulting excited state consists of a mixture of both ny → 3p and π → π* character, which decays with a time constant of 330 fs. In the future, this approach can inform models of VUV photochemistry and aid in designing coherent control strategies for manipulating chemical reactions.
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absorption lines.9 A schematic plot of the relevant diabatic potential energy surfaces of acetone is shown in Figure 1A. Past work has identified that the 3p, 3d, and 4s Rydberg states are in the 7.3−8.2 eV range. In addition, ab initio calculations predict the π → π* valence state to also be important in this region.10 Although the predicted minimum of the π → π* potential energy surface is near 7.4 eV, it enters the Franck−Condon region only above 8.35 eV.11 Multiphoton photoacoustic spectroscopy and resonantly enhanced multiphoton ionization (REMPI) experiments have unambiguously identified the band origins for each 3p state, found at 7.36−7.45 eV,12,13 but the assigned vibrational progressions only extend to 7.76 eV.14,15 Vibrational progressions on 3d states have also been assigned,11 but an underlying quasi-continuous absorption makes these assignments difficult. Extensive theory on the mixing of the 1A1 electronic states11 predicts that the 2 1A1 surface (sometimes labeled the S3 state in other work16,17) is primarily ny → 3py mixed with π → π*, and the 3 1A1 surface is primarily ny → 3dyz in the Franck−Condon region with some π → π* and ny → 3py character. Because acetone has a nearly vertical transition for ionization to the cation ground state, and unperturbed Rydberg states strongly resemble the cation, the long vibrational progressions experimentally observed for these Rydberg states are sufficient to indicate a perturbation of their Rydberg character. Although careful analysis of hot bands, abnormal oscillator strength distributions, and faster-than-expected population transfer have given further evidence for interaction with π → π*,11,18 no direct measurement of the π → π* character of either proposed surface has yet been possible. The dynamics of these states have also been investigated several times in past work, with varying conclusions. REMPI
INTRODUCTION Photoinduced chemical reactions, important throughout nature, must proceed on excited potential energy surfaces. Femtosecond spectroscopic studies of molecular excited-state dynamics have concentrated on the first one or two excited singlet states,1−4 largely due to the challenge of producing short pulses of light in the vacuum ultraviolet (VUV) region to access higher energy surfaces. As a consequence, highly excited states near 7−10 eV remain poorly understood despite their relevance to ionospheric and interstellar chemistry. These environments are bathed in high-energy photons, which cause nonstatistical fragmentation controlled by the fast early time dynamics.5−8 Internal conversion among these highly excited states can deposit enough vibrational energy to access multiple fragmentation channels and induce rapid dissociation, presenting many challenges for identifying which states are involved and how they relax. Moreover, increased understanding of ultrafast vacuum UV photochemistry may enable new strategies for coherent control, because the resulting dissociations can occur faster than internal relaxation processes. New femtosecond VUV light sources have dramatically improved both the flux and wavelength range available for spectroscopy of highly excited states. Specifically, low-order harmonic generation in hollow waveguides allow direct, singlephoton access to almost any wavelength of interest in the VUV, so that experiments do not need to rely on multiphoton schemes that often populate multiple states or leave ambiguities in the number of photons involved. Here we explore acetone as a model system to probe the first steps involved in VUV-induced photochemistry. Acetone is the simplest ketone, and its highly excited states have been extensively studied with both frequency-domain and timedomain spectroscopy. However, there is no clear consensus on the nature of the excited states near 8 eV, where a broad but structured continuum appears under a series of sharp Rydberg © XXXX American Chemical Society
Received: February 3, 2017 Revised: March 6, 2017 Published: March 7, 2017 A
DOI: 10.1021/acs.jpca.7b01112 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 1. (A) Schematic potential energy surfaces of the first several singlet electronic states of acetone and the first two cation states. Each surface is labeled by its dominant character. (B) Experimental apparatus for studying acetone excited by 8 eV photons. This apparatus provides momentumresolved electron−ion coincidence detection.
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METHODS For the single photon ionization experiments, we used an amplified Ti:sapphire laser (KMLabs) producing pulses at a wavelength of 780 nm with 1.5 mJ energy and 35 fs duration at a repetition rate of 10 kHz. Part of the laser output (200 μJ) is used to generate VUV pump pulses by focusing the beam into a xenon-filled hollow waveguide designed for high harmonic generation,25 producing odd harmonics of the driving frequency (5ω, 7ω). To eliminate harmonics higher than 7ω, we send the beam through a 10 cm long region filled with 3 Torr of krypton. The fifth (156 nm) and seventh (111 nm) harmonics are then focused using dielectric mirrors coated for 155 nm (Acton optics), which were found to also reflect a useful amount of 7ω. The ionization potential of acetone (9.7 eV) is between harmonics 5ω and 7ω, so that only 7ω produces ions for these initial single-pulse experiments. For our pump−probe multiphoton ionization experiments, the laser pulse is split into two parts using a beamsplitter. The first part (200 μJ) is upconverted to 5ω and 7ω in the same manner as for the single-photon ionization measurements above, except that a free-standing 200 μm film of SiO2 (Lebow) is used to filter out 7ω and all higher harmonics, allowing only 5ω to pass. The second beam is delayed and upconverted to either 2ω or 3ω using an EKSMA Optics Femtokit. We can choose either of these energies (3.2 or 4.8 eV) to use as a probe pulse to ionize acetone from the excited state. A 625 Hz chopper in the 5ω beam path allows direct comparison of the signal with and without the pump pulse present. To measure electron and ion energies in coincidence, we use an imaging photoelectron−photoion coincidence (PEPICO) spectrometer, shown schematically in Figure 1B, which has been described in detail previously.26 The detectors can measure the position and time-of-flight of the imaged ions and electrons. We record each ion−electron pair individually, keeping count rates low (0.1 counts/pulse average) to avoid false coincidences. Position and time-of-flight information allow us to determine the full 3D momentum vector for each particle.
(2+1) spectroscopy near the 3p band origins indicates that the 3py (2 1A1) ground vibrational level has a lifetime near 100 ps, while 3pz and 3px are at least 1 ns.18 Some femtosecond pump−probe studies near 8 eV have assigned the decay pathways of the excited state, which range from 50 to 330 fs, to molecular fragmentation occurring on the excited-state surface.19,20 Several ultrafast studies show evidence of much faster dynamics in the higher Rydberg levels that quickly fall to the 2 1 A1 state. Very recent work by Hüter and Temps found strong evidence that the 3 1A1 (3dyz) state vanishes faster than their 90 fs pulse, while 2 1A1 lives at least 160 fs.21 The same study also has some evidence that the decay time of the 2 1A1 state depends on the level of vibrational excitation. Rusteika et al. saw photoelectrons consistent with population in the 2 1A1 surface after an 8.8 eV (two-photon) pump resonant with n = 4 Rydberg levels,17 but other photon combinations and dynamics could also explain this signal. Maierhofer et al. accessed n = 6 Rydberg levels at 9.2 eV but consistently observed a photoelectron signal coming from the 2 1A1 surface.16 These combined experimental studies indicate that the 2 1A1 surface may be a bottleneck of the electronic relaxation. Theoretical studies22 suggest that this surface should relax to 1 1B2 (usually called S2), which is known to be the ny → 3s Rydberg state.23,24 Independent of the exact assignments, it is clear that several different states are coupled together after VUV excitation of acetone and that decay times in this energy range are short. As a result, time-domain spectroscopy may be essential to unravel the complex character of these states. In this paper we report time-resolved photoelectron−photoion coincidence (PEPICO) experiments using femtosecond VUV excitation pulses. We find for the first time direct evidence that the 2 1A1 surface in acetone, prepared by 8 eV photons, is actually a strongly mixed state consisting of (ny → 3p) and (π → π*) character that decays with a time constant of 330 ± 30 fs. A reliable assignment of this state required a comparison of single-photon ionization with several pump−probe schema that could access both n−1 and π−1 cation states. B
DOI: 10.1021/acs.jpca.7b01112 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Acetone was introduced into the ultrahigh vacuum chamber through an effusive nozzle with a backing pressure of 1−2 Torr, roughly the vapor pressure of acetone held at −78 °C by a 2propanol/dry ice slurry. The acetone for this experiment was purchased from Fisher Scientific (≥99.5% purity) and used without further purification.
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RESULTS AND DISCUSSION A. Ionization to Cation Ground State with 11.2 eV via One-Photon and Two-Photon Processes. We first discuss the ionization of acetone by two different ionization pathways with the same total energy: (1) ionization by a single 11.2 eV (7ω) photon or (2) a two-step process corresponding to excitation by an 8.0 eV photon (5ω) followed by ionization from the excited state using a 3.2 eV (2ω) photon. Both schemes can produce an acetone ion (58 amu) or an acetyl ion (43 amu) formed by the loss of a methyl radical, because the total photon energy in each ionization scheme is above the appearance potential for the acetyl cation (10.4 eV).27 The onephoton ionization scheme produces an electron kinetic energy spectrum with a single peak at 1.5 eV (Figure 2, pink curves).
Figure 3. Time-resolved photoelectron spectra, separated by coincident ion mass. The dominant signal at 0.8 eV does not change energy with time. The rate of decay, 330 fs, is the same whether the ion fragments or not.
ionization (8 + 3.2 eV) and may include some contribution from a short-lived excited state other than 2 1A1. The lower energy (
