Article pubs.acs.org/JPCA
Superexcited State Dynamics of OCS: An Experimental Identification of Three Competing Decay Channels among Autoionization, Internal Conversion, and Neutral Predissociation Qiaoli Hao,†,‡ Jinyou Long,†,‡ Xulan Deng,†,‡ Ying Tang,†,‡ Bumaliya Abulimiti,*,§ and Bing Zhang*,†,‡ †
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PR China ‡ University of Chinese Academy of Sciences, Beijing 100049, PR China § College of Physics and Electronic Engineering, Xinjiang Normal University, Urumqi 830054, PR China ABSTRACT: The 7sσ and 6pσ superexcited Rydberg states of OCS belonging to series converging onto the B̃ 2Σ+ ionic limit have been successfully prepared by three-photon UV excitation, and their ensuing competing relaxation processes have been probed by a time-delayed IR ionization pulse. The time profiles of S+ ions, which encode their fragmentation mechanism, are only observable at high pump intensities, thus providing unique experimental identification of the neutral predissociation channel producing S* atoms. Benefiting from this feature and by comparison with the time behavior of OCS+ ions, three competing relaxation channels are identified: autoionization associated with both X̃ 2Π and à 2Π ionic states; internal conversion to isoenergetic RA states, the deactivation of which manifests as a picosecond decay in the time profile of OCS+ ions; picosecond neutral predissociation appearing as a nondecaying plateau in the time profiles of S+ ions.
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INTRODUCTION Superexcited states are neutral atomic or molecular excited states with an energy above the first ionization energy. Examples are singly excited Rydberg states converging to internally excited states of the ion, as well as doubly excited or inner-core excited valence states embedded in the ionization continuum.1−3 Because of their special energy locations and relatively high reactivity, superexcited states contribute significantly to not only photophysics4,5 but also photochemical reactions in the upper atmosphere and planetary space.6 As a result, research on the photoionization or photoabsorption cross section and photodissociation mechanism of superexcited states has been extensively carried out by using synchrotron radiation in the extreme ultraviolet range as the excitation source,6 whereas the time-resolved observation of their relaxation dynamics is very rare.7,8 For molecular superexcited states, many relaxation channels are commonly competitive, including fluorescence, relaxation to the continuum of isoenergetic ion states (autoionization), relaxation to lower-lying vibronic states of the neutral molecule (i.e., internal conversion), and reactive pathways resulting in isomerization or fragmentation of the molecule.3 This situation is extremely complicated for larger polyatomic molecules due to their multidimensionality of the increasing internal coordinates, making the experimental observation and identification of their relaxation dynamics much more difficult. Consequently, © XXXX American Chemical Society
superexcited states in polyatomic molecules have rarely been interrogated in the time-resolved domain,2,3,9 especially concerning the identification of their competing decay channels. Only a few cases have been reported to date for time-resolved experiments of diatomic molecules of superexcited nitrogen7 and oxygen8 molecules. Herein, we report a time-resolved observation of relaxation dynamics for superexcited Rydberg states in carbonyl sulfide, as an example of successful identification of competing relaxation channels in polyatomic molecules. In contrast to non-time-resolved experiments6 in which superexcited states are prepared directly from the neutral molecule ground state typically using single photon excitation in the extreme ultraviolet wavelength range, an ultraviolet multiphoton excitation scheme is also utilized to prepare molecular superexcited states.2,3 Because multiphoton excitation requires relatively high pump intensities, intermediate resonant states within the multiphoton excitation is practically unavoidable, especially when higher excited states have large absorption cross sections. However, when time-resolved ion yield spectroscopy is used, such resonant effects can be examined if the pump power dependences are carefully Received: February 27, 2017 Revised: May 2, 2017 Published: May 3, 2017 A
DOI: 10.1021/acs.jpca.7b01895 J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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RESULTS AND DISCUSSION Superexcited Rydberg states of 7sσ and 6pσ have been selectively excited using three-photon UV absorption in this work. The excitation wavelengths are respectively fixed at their origins, which have been unambiguously assigned below the second excited ionic state B̃ 2Σ+, specifically in the range 15.075−16.041 eV.12−14 By finely tuning laser intensities, we have monitored associated dynamics exclusively by using timedelayed IR ionization pulses. The results for the 7sσ state are presented first. For the 7sσ state, three photons of 237.5 nm are required to access its origin. Using a three-photon excitation scheme, both one-photon and two-photon absorption processes might be possible in view of laser intensity and thus unwanted ion signals from intermediate resonant states might be collected as well. For suppressing photon processes of order lower than 3, both pump and probe intensities are delicately tuned, resulting in a series of time profiles of the parent ions OCS+. Time profiles for the parent ions OCS+ are shown in Figure 1, exhibiting a
checked. In combination with a time-delayed infrared ionization pulse, the ensuing relaxation dynamics of superexcited states is clearly probed and distinguished from direct ionization. In this work, we concentrate on relaxation dynamics of superexcited Rydberg states belonging to series converging onto the second excited ionic state B̃ 2Σ+ limit in Carbonyl sulfide, which is the most abundant sulfur-containing gas in the atmosphere and also the simple linear polyatomic molecule. Moreover, to systematically examine the different effects on their relaxation dynamics, superexcited Rydberg states of 7sσ and 6pσ (commonly denoted as B̃ 2Σ+ nlσ) which represent different Rydberg series are specially selected and investigated using transient ion yield spectroscopy coupled with threephoton UV excitation and IR-probe scheme. Although the existence of intermediate resonant states may hamper the effective detection of these superexcited states, we are able to collect the OCS+ and the S+ ions originated exclusively from superexcited optically bright states by finely tuning laser intensities. In particular, the time profiles of S+ ions, which are only observed at relatively high pump intensities, provide a direct observation and identification of the evolution of superexcited B̃ 2Σ+ nlσ states and enable us to determine the time scales for all competing decay channels.
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Article
EXPERIMENTAL SECTION
The time-of-flight mass spectrometer (TOF-MS) has been described in our previous publication.10 The commercial OCS (10% OCS in He) is introduced into the vacuum chamber through a pulsed valve (General Valve, series 9) to create a molecular beam, which is further skimmed before interaction with the lasers. Details of our femtosecond laser system have also been presented.11 Briefly, the output of the regeneratively amplified Ti:sapphire laser (4.5 mJ, 1 kHz, 800 nm) is divided into three arms. One with an energy of 1.5 mJ is used to drive an optical parametric amplifier (Light Conversion, TOPAS-C) producing the pump pulses 237−240 nm, whereas the second fundamental arm with the same energy is used without transformation as the probe pulses. When the probe pulses are passed through a broad-band retroreflector (UBBR1-5I, Thorlabs) mounted on a PC-controlled microdisplacement stage (PI, M-126.CG1), the pump−probe delay time (Δt) is accurately achieved. The pump and probe pulses are respectively focused by two spherical planoconvex lens (CaF2, both with f = 400 mm) and collinearly combined through a dichroic mirror before entering the ionization chamber and subsequently crossing perpendicularly with the molecule beam. Photoions thus generated are accelerated by an electrostatic lens system and then projected onto a pair of chevron microchannel plates (MCPs) located at the end of the 36 cm flight tube. To find the appropriate pump or probe conditions, the pump and the probe intensities are finely tuned with two neutral density filters, respectively. Moreover, the intensities for both the pump and the probe pulses are controlled to be safely smaller than 1012 W/cm2, minimizing the undesirable high-field effects, such as above-threshold ionization (ATI). In addition, the time zero is determined using nonresonant multiphoton ionization of Xe, and the instrument functions are measured to be 170−190 fs in all excitation wavelengths by employing Gaussian fit to a series of temporal traces of the Xe+ ion.
Figure 1. Time profiles of OCS+ ions collected with 237.5 nm excitation. Representative pump and probe energies are indicated. Circles correspond to experimental data, and colored lines indicate individual components. As done by Montero,15 pump background signals in the upper three panels have been subtracted before normalization.
strong dependence on laser intensity. Except for the uppermost panel (a), which is well characterized by a monoexponential decay, all profiles can be well fitted by a biexponential decay convoluted with the measured cross-correlation function. Time constants extracted are summarized in Table 1 and also indicated in associated panels. In panel a, only one decay Table 1. Lifetimes Extracted for the Superexcited B̃ 2Σ+ nlσ States OCS+
B
S+
nlσ
τ/fs
τ2/ps
τ/fs
τPD/ps
7sσ 6pσ
43 49
1.76 1.56
174 127
1.60 1.08
DOI: 10.1021/acs.jpca.7b01895 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A component of ∼116 fs is obtained with a relatively high probe energy of 70 μJ and a relatively low pump energy
