Resonant Auger Decay of Above-threshold Core-excited H 2 O The study of decay processes of core-excited molecules provides a wealth of information on the nature of the intermediate state and on the dynamics of photoexcitation, photoemission and photofragmentation processes. The typical experimental procedure is to identify resonant processes in photoabsorption measurements, and then to tune the photon energy to the position of a particular resonant feature and to investigate the corresponding valence electron emission. The resonant Auger spectrum thus obtained includes features related to final states with one electron vacancy (“participator” decay) or two e l e c t r o n v a c a n c i e s a n d o n e e x c i t e d e l e c t r o n (“spectator” decay). It is also possible to identify features related to fragments in the case of dissociation events occuring on the same time scale of electron decay (the so-called “ultrafast” dissociation). In water, the photoabsorption spectrum around the O K -edge has been measured several times, recently with state-of-the-art resolution [1] . In the above- threshold region, the photoabsorption curve is rather Fig. 1. (a) Total ion yield spectrum of H 2 O in the O 1 s excitation region measured with ~ 50 meV photon bandwidth. (b) Negative ion yields for the H - and O - fragments (curves from Ref. [2]). flat, without pronounced resonant features. H owever, in one particular experiment, namely negative ion yield spectroscopy, it has been possible to identify a resonant feature approximately 10 e V above the ioni z ation threshold [2] . This feature has to be related to doubly excited states, since it is well k nown that there are no shape resonances in water. In F ig. 1 (a), we show the total ion yield spectrum and the angle-resolved ion yield spectra recorded at 0 and 90 with respect to the polari z ation vector. All spectra appear rather flat in the photon energy region j ust above the ioni z ation threshold ( 539 . 79 e V ). At variance with this finding, in F ig. 1 (b) from R ef. [2] , we show partial ion yield spectra of negative fragments O - and H - . It is evident that above threshold the total ion yield spectrum (which we can consider e q uivalent to photoabsorption) is featureless, while a broad resonance at approximately 10 e V above the threshold is clearly evident in the O - negative ion yields. W e recorded resonant Auger spectra at several photon energies within the width of the resonance, locating its position with the aid of negative ion yield spectra [3] . The aim was to characteri z e the possible increase in relative intensity of spectral features related to single-hole final states, and / or to verify the presence of features which could be connected to ultrafast dissociation, i.e. lines derived from resonant Auger decay of the O H fragment rather than from decay of the intact molecule. In a previous wor k , ultrafast dissociation was investigated in water excited below the O K -edge, and the decay spectrum of the O H fragment was well characteri z ed [4] . In F ig. 2 we show resonant Auger spectra recorded at 0 and 90 , I( 0 ) and I( 90 ), respectively, with respect to the polari z ation vector and the angle-integrated spectrum as given by I( 0 ) +2 I( 90 ). The photon energy corresponds to the maximum of the resonance identified in the negative ion yield curves. The decay spectra exhibit a broad feature at a binding energy of 32 . 2 e V , which is related to an inner-valence molecular state, and superimposed to it some sharp structures with vibrational substructure. The relative position of such features and their vibrational spacing are consistent with the resonant Auger spectrum of the O H fragment [4] . As a further test, we measured the decay spectrum at several different photon energy values within the resonant structure in the absorption. W e can confirm the assignment of such pea k s as due H – O – TIY I(0 ゜ ゜ ) I(90 ) 545 550 535 540 3 45 4 5 2b 2 np b 2 4a 1 np b 1 /a 1 (a) Photon Energy (eV) (b) Intensity (arb. units) 81 References [1] A. Hiraya et al. : Ph y s . Rev . A 63 (200 1 ) 042705 . [ 2 ] W . C . Stolte et al. : Ph y s . Rev . A 68 (2003) 02270 1. [ 3 ] M . N . P ia nc a stell i , R . S a nk ari , S . So r ensen, A. De F a n i s, H. Yosh i d a , M . K i t a j i m a , H. T a n a k a a nd K . Ued a : to be publ i shed . [ 4 ] I . H jelte et al. : Chem . Ph y s . Lett . 334 (200 1 ) 1 5 1. [ 5 ] H. Oh a sh i et al. : Nucl . Inst r um . Meth . A 467 (200 1 ) 533 . M . N . P ia nc a stell i a , R . S a nk ari b a nd K . Ued a c ( a ) Dept . of Chem i c a l Sc i ences a nd Technolog i es, Un i ve r s i t y “To r Ve r g a t a ”, It a l y (b) Dept . of Ph y s i c a l Sc i ences, Un i ve r s i t y of Oulu, F i nl a nd (c) Inst i tute of Mult i d i sc i pl i n ary Rese ar ch fo r A dv a nced M a te ria ls, Tohoku Un i ve r s i t y E-m ai l: p ia nc a @ r om a 2 .i nfn .i t Fig. 2. Resonant Auger spectrum recorded on top of the resonance at ~ 550 eV photon energy [see Fig. 1(b)]. to the dec ay of the O H ra d i c a l, a s i nd i c a ted i n F i g . 2, on the g r ounds of the ir d i spe r s i on l a w . N a mel y , wh i le the molecul ar st r uctu r e d i spe r ses l i ne ar l y i n k i net i c ene r g y a s a funct i on of photon ene r g y , thus r em ai n i ng a t const a nt b i nd i ng ene r g y , the sh ar p st r uctu r es show a pp ar entl y const a nt k i net i c ene r g y. Such beh a v i ou r i s t y p i c a l of spect ra l fe a tu r es r el a ted to f ra gments [ 4 ]. We c a n conclude th a t the r eson a nt st a te ev i dent onl y i n the neg a t i ve i on yi eld spect ra i s d i ssoc ia t i ve, a s p r oven b y the a ppe ara nce of f ra gment- r el a ted spect ra l st r uctu r es . The ve ry h i gh spect ra l i ntens i t y a nd r esolut i on a v ai l a ble a t SP ri ng-8 h a ve en a bled us to detect such a “h i dden” r eson a nt p r ocess . The expe ri ment h a s been c arri ed out on the c b ra nch of the soft-X- ray photochem i st ry BL27SU [ 5 ]. The ra d ia t i on sou r ce i s a f i gu r e-of-e i ght undul a to r th a t p r ov i des l i ne ar l y pol ari zed l i ght: the pol ari z a t i on vecto r E i s ho ri zont a l fo r the f ir st-o r de r h ar mon i c l i ght a nd ve r t i c a l fo r the 0 . 5-o r de r h ar mon i c l i ght . The monoch r om a to r i nst a lled on th i s b ra nch i s of H ett ri ck t y pe a nd p r ov i des monoch r om a t i c soft X- ray s w i th the b a ndw i dth ≈ 50 meV i n the O 1 s exc i t a t i on r eg i on . I(54.7) I(0) I(90) 38 36 42 40 34 32 30 2a 1 -1 1 Σ + 1 ∆ Photon Energy (eV) Intensity (arb. units) 82
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