2 June 2000

Ž .Chemical Physics Letters 322 2000 491–495www.elsevier.nlrlocatercplett

A selective photo-induced reaction in the ion-moleculecomplex Mgq–FCH 3

Xin Yang, Yihua Hu, Shihe Yang)

Department of Chemistry, The Hong Kong UniÕersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Received 26 January 2000; in final form 9 March 2000

Abstract

q Ž . Ž .We have produced cation–molecule complexes Mg – FCH ns0–6 in a laser-ablation pickup source. A photo-in-3 n

duced reaction in Mgq–FCH was observed for the first time, with an exclusive production of CHq and MgF. This reaction3 3Ž q. Ž q .selectivity is remarkable considering the fact that other product channels CH q MgF and Mg qFCH are much3 3

more energetically favorable. We have measured the relative photodissociation product yield as a function of the excitationwavelength in a broad spectral region. The reaction mechanism is considered to involve a curve crossing to an ionic PES,leading to the formation of the final products CHq and MgF. q 2000 Elsevier Science B.V. All rights reserved.3

In recent years, the photo-induced reaction in apre-formed complex has received considerable inter-

w xest 1–3 . This type of transition state spectroscopyw xhas been extensively applied to negative ions 4,5 ,

w xneutral complexes 6–11 , and surface-alignedw xspecies 12 in both frequency and real time domains

w x13 . Photodissociation spectroscopy of metalcation–molecule complexes is potentially a usefulmeans to explore the potential energy surfaces in the

w xtransition state regions of excited state reactions 14 .Previous studies on photodissociation of metalcation–molecule complexes were mainly directed to

w xunderstand the solvation phenomenon 15–18 . Morerecently, photo-induced reactions were observed incomplexes of metal cations and solvent molecules

) Corresponding author. Fax: q852-2358-1594; e-mail:chsyang@ust.hk

w x19–22 . In this letter, we report a highly selectiveŽ q qphoto-induced reaction Mg –FCH qhn™CH q3 3

.MgF in a cationic complex for the first time in abroad spectral region.

The main experimental setup has been describedw xelsewhere 23,24 . Briefly, the metal cation–mole-

q Ž .cule complexes Mg – FCH were produced in a3 n

pickup ion source, which consists of a pulsed nozzleand a laser-ablation target. The FCH gas seeded in3

Ž .argon FCH rAr ;20% expanded supersonically3

through a 0.5 mm diameter orifice of the pulsedvalve operated at a backing pressure of ;40 psi.

ŽThe laser beam of an Nd:YAG laser 532 nm; ;30.mJrpulse was weakly focused on a sample disk.

The laser-vaporized species dominated by metal ionsand atoms travelled perpendicular to the supersonicjet stream, and were picked up by cluster species inhigh density regions of the supersonic jet. The nascentcomplexes were carried by the expanding stream to

0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 00 00457-7

( )X. Yang et al.rChemical Physics Letters 322 2000 491–495492

the extraction region of a reflectron time-of-flightŽ .mass spectrometer RTOFMS . After vertical extrac-

tion, the cation–molecule complexs were selectedwith a pulsed high-voltage deflection type mass-gate.Photodissociation was carried out using an excimerpumped dye laser in the ion turn-around region of

w xthe reflectron assembly 25 . Thereafter, both theparent and fragment ions were accelerated again andmass analyzed based on their flight time through thesecond field-free region of the RTOFMS.

A typical mass spectrum of the MgqqFCH 3

system from the pick-up source is displayed inFig. 1. The metal-bearing mass peaks can be readilyrecognized from the characteristic isotope distribu-

w xtion of magnesium 26 . As shown in Fig. 1, twoseries of mass peaks are identifiable. The series ofhigher intensity is assigned to the cation-cluster com-

q Ž .plexes Mg – FCH , and the other of lower inten-3 nq Ž .sity to the reaction products MgF – FCH . The3 n

q Ž .Mg – FCH cluster species are obviously the3 n)1

association products, which were presumably formedq Ž .from collisions between Mg and FCH . These3 n

cation–molecule complexes could be stabilized byeither a third-body collision or by evaporation of

Ž .FCH from FCH . Their intensities could be con-3 3 n

trolled by adjusting the concentration of FCH in the3

argon carrier gas and the pulse width of the gasvalve. The other series is thought to be from thecollision-induced reactions. Since the reactionŽ q q .FCH qMg ™MgF qCH is endothermic by3 3

Fig. 1. Time-of-flight mass spectrum of the MgqqCH F system3

from the pick-up nozzle source.

Fig. 2. Photodissociation difference mass spectrum of Mgq–FCH3

at 320 nm.

only 0.4 eV 1, the kinetic energy of the Mgq ionsfrom laser ablation is more than enough to overcomethe reaction barrier if the collision complex is notstabilized during its lifetime.

Fig. 2 presents a photodissociation difference massspectrum recorded after the mass-selected parentMgq–FCH was excited at 320 nm. The negative-3

going peak on the high mass side signifies a deple-tion of the mass-selected parent due to photofrag-mentation, and the positive-going peak indicates the

Ž q.appearance of a daughter ion in this case, CH 3

from the photofragmentation process. The negative-going parent peak has a relatively poor mass resolu-tion because the extractor and reflectron voltagesettings were optimized for the maximum overlapbetween the photolysis laser beam and the mass-selected parent ion packet in the turn-around regionof the reflectron assembly. The better mass resolu-tion of the daughter ion peaks can be ascribed to thesmall size of the photolysis laser beam and thereduced kinetic energies of the daughter ions. Themost striking observation is that only a single disso-ciation product ion CHq is observed in the photoly-3

q Ž .sis of Mg –FCH Fig. 2 . Particularly remarkable3

is that this photodissociation channel is not energeti-cally favorable in comparison with other channels.

1 Ž .Calculated from the bond dissociation energies BDE ofq Ž qF–CH and Mg –F the BDE of Mg –F is based on our ab3

.initio calculation results .

( )X. Yang et al.rChemical Physics Letters 322 2000 491–495 493

Fig. 3. Energetic diagram for relevant species possibly involved inthe photo-induced reaction in Mgq–FCH . The data are taken3

w xfrom Ref. 26,29 as well as from our ab initio calculations.

Ž q .For example, as shown in Fig. 3, the CH qMgF3

channel is energetically less favorable by as much asŽ q .;2 eV than the channels of MgF qCH and3

Ž q . qMg qFCH . Our observation on Mg –FCH is3 3

also in contrast to the results of previous photodisso-ciation experiments on other cation–molecule com-

w xplexes 27–29 . For example, in the photodissocia-q Ž . w xtion of M -CH OH Msalkaline metal 28,29 ,3

Mq is the most prominent fragment. CHq was3

observed only with a very low intensity comparedwith those of the dominant photofragments Mq andMOHq.

In carrying out our photodissociation experiments,Žthe dissociation laser fluence was kept low -1

2 .mJrcm to minimize multiphoton processes. Laserfluence dependence of the fragment ion indicated aone-photon process. The photodissociation actionspectrum was taken in this one-photon regime. Thephotodissociation action spectra of the Mgq–FCH 3

complex in the wavelength region 230–390 nm aregiven in Fig. 4. The daughter ion signal intensity wasnormalized against both the parent ion signal inten-sity and the dissociation laser fluence. The spectrawere collected in segments and the different seg-ments were pieced together by normalizing the spec-tra at the overlapping edges of the laser dye ranges.There are two pronounced peaks in the photodissoci-ation action spectrum of Mgq–FCH as shown in3

Fig. 4. These two peaks are centered at around 255nm and 320 nm, which are at the blue side and red

q Ž 2 2 .side of the Mg 3 P§3 S atomic transition, re-spectively. It should be pointed out that the peakheight in Fig. 4 is associated with the relative inten-sity of a specific fragment ion. It depends not onlyon the oscillator strength of the transition to theexcited state but also the probability for the excitedstate to find its way to the exit channel, in which thefragment ion emerges.

Although the 32 P states of Mgq are nearly degen-erate, they are expected to be splitted by the presenceof FCH . This splitting can be large due to the3

strong interaction between Mgq and FCH . We3

believe that the cation–molecule complex has aground state structure, in which the Mgq cation isattached to FCH through the F atom. The observed3

reaction products CHqqMgF and the corresponding3

photodissociation action spectrum are consistent withthis structure. As shown in Fig. 3, the 32 P states maybe splitted into at least two components: one is withthe p orbital parallel to the bond axis of Mg-F, andz

the rest are with the p orbitals perpendicular to thex , y

bond axis of Mg-F. The parallel obital orientation isexpected to have a higher energy than that with aperpendicular orbital orientation. Therefore, the two

Žpeaks of the photodissociation action spectrum Fig..4 can be assigned to these two transitions: the red

peak corresponds to the transition to 3P , and thex , y

blue peak to the transition to 3P .z

There is one legitimate concern about the possibil-ity of the parent complex being of the structural form

qŽ . qŽ .MgF CH instead of Mg FCH . However, our3 3

findings so far seem to support the latter structure.

Fig. 4. Photodissociation action spectrum of Mgq–FCH . The3q Ž 2 2 .wavelength of the Mg 3 P§3 S atomic transition is shown

by a dotted line.

( )X. Yang et al.rChemical Physics Letters 322 2000 491–495494

qŽ .First, the species MgF CH , if present, must come3

from the recombination of the thermal reaction prod-ucts MgFq and CH . In this situation, the abundance3

of MgFq should be much higher than that ofqŽ .MgF CH , which is contrary to the mass spectral3

Ž .observation Fig. 1 . Second, the action spectrumŽ .Fig. 4 shows clearly that the peaks between the

q Ž 2 2 .atomic transition Mg 3 P§3 S are derived fromthe excitations centered at Mgq, but perturbed signif-icantly by the presence of FCH . On the other hand,3

q qŽ .MgF is a closed shell species in MgF CH , and3qŽ .therefore the excitation spectrum of MgF CH 3

must originate from CH , which should be qualita-3

tively different from the spectrum we observed. Fi-nally, our preliminary ab initio calculation at theMP2 level suggests that the ground state minimumenergy structure of the complex should be Mgq–

˚FCH with the F–C bond extended by ;0.1 A3˚Ž .relative to that of the free FCH . 1.38 A The angle3

enclosed by Mg–F–C is 179.78, and this approxi-mately linear complex geometry is remarkably con-sistent with overlap of the 3P peaks in the actionx , y

Ž .spectrum Fig. 4 .The exclusive formation of the energetically less

favorable photo-product CHq in such broad spectral3

region is thought-provoking. The excited state dy-namics must have played a crucial role in the photo-induced reaction although the detailed mechanismremains to be investigated. As in photo-induced reac-

w xtions of Na–FCH 9 , a charge-transfer mechanism3

is likely to be involved in the photo-induced reactionof Mgq–FCH . However, it is not possible for an3

electron to jump from the metal atom to the halideŽ .due to the extremely high ionization energy IE of

q Ž . w xMg 15.035 eV 26 and a large negative electronŽ . w xaffinity of FCH y6.2 eV 30 . Such an electron3

jump would lead to the formation of MgFq andCH , which was not observed. On the other hand, it3

is also difficult for FCH to transfer an electron to3Ž q.Uthe excited Mg due to the very high IE of FCH 3

Ž . w x12.47 eV 26 . What is happening may be a con-certed action of the two processes mentioned above.Specifically, when the outermost 3s electron of Mgq

is excited to 3p, the electron gains a sufficient energyso that the electron cloud spreads out gradually toFCH , rendering the F–C bond to extend. As the3

F–C bond becomes longer, the electron in FCH is3

more easily ionized because CH has a much lower3

Ž . Ž . w xIE 9.83 eV than FCH 12.47 eV 26 . This results3

in a gradual electron transfer from XCH to Mgq 3s,3

which would facilitate the electron transfer fromMgq 3p to FCH . This synergetic forward and3

backward electron transfer would result in an inter-w q y qxmediate Mg –F –CH , in which the F–C bond is3

much extended. Before MgF finds itself having alower IE than CH , the CHq moiety may have3 3

already been released. Therefore, the final outcomeof the reaction would be the formation of MgF andCHq instead of MgFq and CH . We are performing3 3

calculations on the ground state structure and rele-vant potential energy surfaces as well as more exten-sive experiments on the complex and its congeners.Measurement of the kinetic energy release of thephoto-product CHq will be particularly penetrating.3

This will provide more insight into the mechanism ofthis intriguing photo-induced reaction.

q Ž .The cation–molecule complexes Mg – FCH 3 n

have been produced by using the pickup technique.Ž qA highly selective photo-induced reaction Mg –

q.FCH qhn™MgFqCH has been observed for3 3

the first time in Mgq–FCH . We have measured the3

relative photodissociation product yields as a func-tion of the excitation wavelength in a broad spectralregion. The photodissociation action spectrum con-sists of two broad peaks between the atomic transi-

qŽ 2 2 .tion of Mg 3 P§3 S .

Acknowledgements

This work is supported by an RGC grant adminis-tered by the UGC of Hong Kong.

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