A1Σ+State Lifetim e and Predissociation of CuH

De-ping Zhang,Qiang Zhang,Bo-xing Zhu,Yang Chen,Dong-feng Zhao

Hefei National Laboratory for Physical Sciences at the M icroscales,Department of Chem ical Physics, and Synergetic Innovation Center of Quantum Information&Quantum Physics,University of Science and Technology ofChina,Hefei230026,China

A1Σ+State Lifetim e and Predissociation of CuH

De-ping Zhang,Qiang Zhang,Bo-xing Zhu,Yang Chen?,Dong-feng Zhao?

Hefei National Laboratory for Physical Sciences at the M icroscales,Department of Chem ical Physics, and Synergetic Innovation Center of Quantum Information&Quantum Physics,University of Science and Technology ofChina,Hefei230026,China

A combined cavity ringdown(CRD)and laser induced fluorescence(LIF)spectroscopic study on theA1Σ+-X1Σ+transition of CuH has been presented.The CuH molecule,as well as its deuterated isotopologue CuD,are p roduced in a supersonic jet expansion by discharging H2(or D2)and A r gasm ixtures using two copper need les.Diff erent profi les of relative line intensities are observed between themeasured LIF and CRD spectra,providing an experimentalevidence for the predissociation behavior in theA1Σ+state of CuH.The lifetimes of individual upper rotational levels arem easured by LIF,in which theJ′-dependent predissociation rates are obtained.Based on the previous theoretical calculations,a predissociation mechanism is concluded due to the strong spin-orbit coup ling between theA1Σ+state and the lowest-lying triplet3Σ+state,and a tunneling effectmay also be involved in the predissociation.Sim ilar experiments are also performed for CuD,show ing that theA1Σ+state of CuD does not undergo a predissociation process.

CuH molecule,Laser induced fluorescence,Predissociation

I.INTRODUCTION

M any transition metal hydrides have been identified in theatmospheresof coolstarsvia observationsof their electronic transitions in the optical range,attracting great interest to chem ical physicists in the laboratory experimental studies on their electronic spectra and chem ical properties[1?3].The copper hydride(CuH) molecule has been identified in sunspot spectra[4]and possibly in the spectrum of the star Piscium 19[5].In addition,because of the sim plest closed d-shell structure in the electronic configuration,CuH represents a prototypical system for a detailed understanding of the transition m etal-hydrogen bonding properties.

Laboratory experiments have been performed to study the electronic transition spectra of CuH,resulting in accurate values of spectroscopic constants of lowlying electronic states[6?9].For theA1Σ+-X1Σ+electronic transition of CuH,a remarkable phenomenon of rotational line intensity distributions in the em ission spectrum of the(0,0)band was found by Sch¨uleret al. in 1939[10],and has led to a long discussion on the nature of this phenomenon in the past century[8,11?14]. In Sch¨uler’s experim ent,CuH m oleculeswere produced and excited in a hollow cathode discharge lam p.It is noted that the intensity of the P(1)line is predom inant in the band,and intensities of all other rotationallines rapid ly decrease w ith increasingJ′.Sch¨uleret al. suggested that this anomalous intensity distribution is due to formation processof CuH moleculesunder a hollow cathode discharge condition.Later,Herzberg and M undie[11]proposed an entirely diff erent exp lanation that the anom alous intensity probably results from p redissociation of theA1Σ+state,via an intersystem crossing w ith a3Σ+state that can be directly dissociated to atom ic H and Cu.This also exp lains the anom alously strong P(1)line,for which the upper(A1Σ+,J′=0) rotational level does not interact w ith a3Σ+state according to K ronig’s selection rules[15].Hulth′en[12] presum ed the anomalous intensity was caused by predissociation of theA1Σ+state into the continuum ofX1Σ+state.In the follow ing experimental studies[8, 13,14],this question still kept disclosed.Both exp lanations,chem ical formation andA1Σ+state predissociation,have got additional support from themeasurement of theB3Π-X1Σ+transition of CuH and/or the electronic spectrum of CuD.In addition,in a recentab initiostudy by M arian[16],the predissociation of theA1Σ+state to a3Σ+statewas predicted.Based on the detailed calculationson thepotentialenergy surfaces,as well as transition dipole mom enta and spin-orbit coup lingsof the low-lying electronic statesof CuH,it issuggested that the lowest-lying3Σ+state,which undergoes an avoided crossing w ith a higher-lying3Σ+state,may be responsible for the experim entally observed intensity anomalies.

In this contribution,we present a combined cavity ringdown(CRD)and laser induced fluorescence(LIF) spectroscopic study on theA1Σ+-X1Σ+transition ofCuH,aim ing for amore in-depth understanding on the nature of the intensity anomalies.Here,CRD isused to record direct absorption spectra w ith relatively reliable line intensities,and LIF is used tom easure the lifetimes of individualA1Σ+state rotational levels.The latter measurement allows us to confi rm the intensity anomalies of CuH to be due to a slow and rotation-dependent predissociation process in theA1Σ+state.

II.EXPERIM ENTS

The experiment is performed using our supersonic jet setup that has been described in detail elsewhere [17?19].In brief,CuH or CuD molecules are produced by a corona dischargeofa gasm ixtureof～3%H2or D2diluted in argon using two copper need les as electrodes. The gasm ixture w ith a stagnation pressure of～6 bar is supersonically expanded into a vacuum chamber via a pulsed nozzle(General Valve,Series 9,0.5 mm orifice).Two copper need les are m ounted on a specially m achined Teflon p late w ith～1 mm spacing,～1.5 mm downstream from the nozzle orifice and perpendicular to the jet expansion.High voltage pulses(～2000 V,～20μs)are app lied to the copper need les to ignite the discharge,producing CuH or CuD molecules in the jet for spectroscopic studies.

Two diff erent spectroscopic techniques,CRD and LIF,are em p loyed.A pulsed dye laser(Sirah,Cobrastretch)pum ped by a third harmonic output of a Nd: YAG laser(Spectra Physics,Lab-190)is split into two laser beam s to record the CRD and LIF spectra,respectively.In the lattermeasurement,the laser beam, which is attenuated to～10μJ/pulse by a neutral density fi lter,is injected into the vacuum chamber.The laser beam is aligned perpendicularly crossing the gas jet and～30mm away from the copper needles.Fluorescence em itted by the laser excited CuH/CuD molecules is collected by a telescope lens system and detected by a photomultip lier tube(PMT)(Hamam atsu,R928).The output signalof the PMT is digitized in an oscilloscope (Keysight,DSOX2024A)and then transferred to a personal com puter for further processing.

The CRD measurement is performed in an optical cavity,which is formed by two highly reflective p lanoconcavem irrorsm ounted on precision alignm ent tools,～60 cm apart on opposite sides of the same vacuum chamber as for LIF measurement.The cavity axis is aligned perpendicularly crossing the gas jet at the same position as LIF excitation beam.The angle between the cavity axis and the LIF excitation laser beam is designed to be～10?.Three setsofhighly reflective cavity m irrors are used in the present experiment,w ith reflectivityR≈99.96%at 385 nm,R≈99.98%at 405 nm,andR≈99.98%at 440 nm,respectively.In order to ensure them irrors retaining their high reflectivity throughout the p lasm a discharge,sm all A r flow is injected in front of the reflective surface of each m irror,acting as a protective gas curtain.The ringdown decay signal is detected by a PMT detection,which is digitized in an oscilloscope.

The whole experiment runs at 10 Hz and a pulse generator is used to guarantee that gas and discharge pulses,as well as laser pulses,coincide in time to obtain the best signal intensity.A LabVIEW program is used to record the CRD and LIF spectra,wavelength tuning of the dye laser,as well as themeasurement of fluorescence decay traces.The absolute frequency of the dye laser output is simultaneously calibrated by a wavelength meter(Coherent,Wavemaster)during the experiment running,which provided a frequency accuracy better than 0.04 cm?1in the recorded spectra.

III.RESULTS AND DISCUSSION

We havem easured theA1Σ+-X1Σ+(0,0)and(1,0) band spectra of CuH,locating at～429 and～401 nm, respectively,by both LIF and CRD techniques to get a straightforward view of rotational line intensities.Fig.1 shows a com parison of the recordedA1Σ+-X1Σ+(1,0) band spectra by LIF and CRD.The observed transition frequencies for individual lines,are in good agreement w ith the previously reported spectroscopic constants [9].However,we notice that,relative line intensities between spectra recorded by LIF and CRD are rather diff erent.It can be clearly seen from Fig.1 that,compared to the CRD spectrum,individual line intensities in the LIF spectrum decreasemuch m ore rapid ly w ith increasing upper rotational quantum numberJ′.This behavior is the same as that has been found by Sch¨uleret al.[10].

Because the LIF and CRD spectra are simultaneously recorded w ith the same CuH source,the different line intensities shown in Fig.1 cannot be exp lained by the chem ical formation process in the discharge source.A simulation of theCRD spectrum using thePgopher software[20]shows that,the overall band profi le recorded by CRD can bewell reproduced w ith a rotational temperature of～50 K,i.e.,the rotational populations of the ground state CuH molecules in our supersonic jet expansion follow a Boltzm ann population.Because the derived ground state population is also app licable for the LIF spectra,observed intensity anomalies can only be exp lained by other dynam ical processes in the upperA1Σ+state.

Since the spectra intensity of LIF can be significantly affected by additional loss of fluorescence em itters via non-radiative processes in the upper state,such as predissociation or internal conversion to other states, know ledge on the upper state lifetimes can provide additional information to understand the observed intensity anom alies.This is realized by recording the decay tracesof the time-resolved fluorescence signalsw ith an average of 1000 pulses.FIG.2 shows two typical fluorescence signals recorded by excitation of P(1)and P(2)lines of theA1Σ+-X1Σ+(0,0)band,respectively, where the background discharge glow and laser scat-tering have been subtracted.A mono-exponential fi t on the falling edge of each recorded fluorescence decay curve results in a fluorescence decay tim e of～99 ns for P(1)transition and～24 ns for P(2)transition,respectively.Since themeasurement is performed in the collision-free zone of a supersonic jet,the derived fluorescence decay tim e is the sam e as the upper state radiative lifetim e,i.e.,the lifetimes(τ)ofJ′=0 and 1 rotational levels in theA1Σ+v′=0 state are 99±5 and 24±5 ns,respectively,where the uncertainty is lim ited to the pulse duration of our dye laser system.

FIG.1 LIF and CRD spectra of theA1Σ+-X1Σ+(1,0) band of CuH.The insert shows the zoomed-in of63/65Cu isotope resolved P(1)line in both spectra.

FIG.2 Typical fl uorescence signals of P(1)and P(2)transitions in theA1Σ+-X1Σ+(0,0)band of CuH.The red traces represent them ono-exponential fi ts,from which upper state lifetim es(τ0andτ1)are determ ined.Errors given in figure are one standard devitation of the fi ts,which is much sm aller than the pulse duration of the dye laser used in the present experiment.

We have measured upper state lifetimes for rovibrational levels w ithJ′=0?3,as p lotted in FIG.3. For higherJ′levels,the fluorescence decay time is found to be comparable to or smaller than the laser pulse duration,and thus cannot be accurately measured.No obvious diff erences are found between the m easured lifetimes for the two copper isotopologues,63CuH and65CuH.On the other side,as shown in FIG.3,a significantJ′-dependence can be clearly seen that that 1/τincreases approxim ately linearly w ith increasingJ′value for bothA1Σ+v′=0 andv′=1 state.Furthermore,1/τofJ′=0 has alm ost an identical value for bothv′=0 andv′=1 levels.This further confi rm s Herzberg and Mundie’s suggestion[11]that theJ′=0 level is not affected by non-radiative processes in the upper state. According to K ronig’s selection rules[15],only states w ith the same valueJand the same symmetry can interact w ith each other.But for3Σ+state,the only level w ithJ′=0 has the opposite symmetry to that of theJ′=0 level ofA1Σ+state.

FIG.3 P lots of 1/τversus rotational quantum numberJ′for both CuH and CuD.

TABLE I TheA1Σ+state p redissociation ratekpreof CuH.

Since theJ′=0 level isnot predissociative,weareable to calculate theJ′-dependent dissociation rate for theA1Σ+state of CuH using the follow ing equation:

wherekpreis the predissociation rate in unit of ns?1, andτnat=τ(J′=0)is the upper state natural lifetime. The derived predissociation rates for theA1Σ+state of CuH are summ arized in Table I.

In a sim ilar way,we have also measured theA1Σ+-X1Σ+(0,0)and(1,0)band spectra of the deuterated isotopologue CuD.The overall profi les of the resulting CRD and LIF spectra are found to be nearly identical. The measured fluorescence decay times for individual linesw ithJ′≤5 in both(0,0)and(1,0)bands of CuD are allw ithin the range of 105±5 ns,and are independent on the upper state rotationalquantum numberJ′. FIG.3 shows a com parison of theA1Σ+state lifetimes between CuH and CuD.It is noticed thatJ′=0 levels of CuH and CuD have alm ost the sam e lifetim e,indicating that theA1Σ+state of CuD does not undergo a predissociation process.

In Ref.[16],it is suggested that the 13Σ+state lying nearby theA1Σ+state is a bonding state and undergoes an avoided crossing w ith a higher-lying 23Σ+state,resulting in a barrier in the dissociation pathway of the13Σ+state.Since theexcitation energiesofA1Σ+v′=0,1(23311 and 24922 cm?1)states are significantly below the calculated barrier of the 13Σ+state,diff erent behaviors between CuH and CuD m ay be due to a tunneling eff ect in the predissociation.This hypothesis is consistentw ith a common understanding that the tunneling eff ect of a hydrogen atom can be orders of magnitude faster than a deuterium atom.To test this, we have also extended the fluorescence lifetime measurement to higher ro-vibrational levels,follow ing the previously reported transition frequencies of hot bands in the wavelength region of 375?440 nm[9].The results are summarized in Table II.Unfortunately,these results do not provide either positive or negative supports in the existence of a dissociation barrier in the 13Σ+state,or a tunneling eff ect in the predissociation. For CuH,themeasured lifetimes ofA1Σ+v′≥2 levels involve severe perturbations of other electronic states, such asB3ΠandC1Πstate,which have been discussed in previous spectroscopic studies[7?9].Such perturbations do not allow for straightforwardly evaluating the change of state lifetim es around the dissociation barrier.Further,previousab initiocalculations by M arian [16]have shown that,wavefunctions of low-lying electronic excited states of CuH are strongly m ixed w ith each other via spin-orbit coup ling,yielding alm ost all the low-lying triplet states are of a singlet-state character.Such couplings do not allow for qualitatively modelling themeasured upper state lifetimes using the previously reported perturbations[7?9].On the other side,the nearly constant lifetimes for CuDv′=0?5 levelsmay be due to a deuteration eff ect that the strength of the avoided crossing between 13Σ+and 23Σ+states are much weaker than that in CuH,yielding a much higher predissociation barrier in CuD.

TABLE II The lifetimes(ns)ofA1Σ+(v′≥2)state rotational levels for both CuH and CuD.

IV.CONCLUSION

The diff erence of relative line intensities in them easured LIF and CRD spectra has provided an experimental evidence for the prediccociation process in theA1Σ+stateof CuH.Based on the lifetimesof individual upper rotational levelsmeasured by LIF,and combined w ith previous theoretical calculations by M arian,the predissociation m echanism is concluded due to strong spin-orbit coup ling between theA1Σ+state and the lowest-lying trip let3Σ+state,and a tunneling effect may also be involved in the predissociation.In contrast, theA1Σ+state of CuD m olecule does not undergo a predissociation process.

V.ACKNOW LEDGM ENTS

This work is financially supported by the National Basic Research Program of China(No.2010CB923302 and No.2013CB834602),the National Natural Science Foundation of China(No.21273212,No.21173205,and No.91127042),the FundamentalResearch Funds for the Central Universities and Chinese Academy of Sciences (No.KJCX 2-YW-N24).

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ceived on February 10,2017;Accepted on March 20,2017)

?Authors to whom correspondence shou ld be add ressed.E-m ail: yangchen@ustc.edu.cn,dzhao@ustc.edu.cn