L ike C O , N O on P t ( l l l ) desorb s as a sin g le peak w ith the tem perature o f peak m axim u m sh iftin g to lo w er tem peratures w ith in creasin g co v e ra g e (4 0 5 K at O .IL to 3 6 0 K at 2 L [3 8 ]). N O is adsorbed n o n -d isso c ia tiv e ly as d eterm in ed by E EL S [3 8 ], X P S [39] and U P S [40] so the d ecrease is a ssig n ed to a d ecrease in Ed w ith increasin g covera g e. O ther authors also o b serve a high tem perature sh ou ld er to this m ain peak. C am p b ell e t al. [41] attribute this to the p resen ce o f a sm all fraction o f surface d efects such as steps. G land and S exton [42] con cu r o b serv in g n ew EELS bands w h ich c o in c id e w ith the m inor peak but further su g g est that so m e o f the N O in this peak is d isso c ia tiv e ly adsorbed as it ex c h a n g ed o x y g e n w ith pre-adsorbed '* 0 y ield in g N'^O and N'^O T P D peaks. D isso cia tio n w as fou n d to be p o ssib le from this strongly bound P2 peak as ev id en ced by d esorption o f sm all am ounts o f N2 at ~ 4 7 5 K and O2 at ~ 8 6 0 K (< 2% o f the total desorption [4 1 ]). A lo w tem perature (2 0 0 K ) desorption peak w as seen in the w ork o f G land and S ex to n w h ich w as not exp la in ed in the d isc u ssio n but is m ost lik e ly the result o f m u ltilayer adsorption. T h e results o f C om rie et al. [43] are in stark contrast to th ose d isc u sse d ab ove. T h ese authors ob served sig n ifica n t d isso cia tio n o f N O on P t ( l l l ) - ap proxim ately tw o-thirds d isso cia ted lea v in g the surface as N2 and O2 on heating. H o w ev er, this m ay be ex p la in ed by the fact that the P t(l 11) surface used w as not a sin g le crystal surface but a p o ly cry sta llin e ribbon o f m o stly (1 1 1 ) orientation.
A g ain , like C O , desorption o f N O from stepp ed surfaces results in tw o T D S states [43, 4 4 ]. On P t( llO ) C om rie e t al. fou n d the m ore stro n g ly bound P2 state occu p ied first and w h en this w as appreciably fille d the m ore w e a k ly bound Pi state started to fill. S o m e d eco m p o sitio n also occurred but this w as su bstan tially less (< 20% ) than th ese authors o b served on their (1 1 1 ) surface (= 66% ). T h e P t [ 1 2 ( l l l ) x (1 1 1 )] surface sh o w ed a m ore p ron oun ced terrace peak w h ich d om inated at saturation co vera ge. In this w ork the occup ation seq u en ce w a s as ob served on P t( llO ). T he step peak d om inated at saturation co v e ra g e acco u n tin g for 57% o f the total desorption. T his im p lie s the con centration o f N O m o lec u le s on the step sites is 2 .6 tim es greater than on the terraces.
A p(2x2) LEED pattern was observed by Ibach and Lehw ald [38] and Hayden [45] when P t ( l l l ) was saturated with NO at low tem peratures. The same pattern was observed by K iskinova [39] following room tem perature saturation. A reasonable (2x2) pattern was obtained by G land and Sexton on P t ( l l l ) at lOOK which did not persist above 300K. Com rie et al. found no significant change in the (1x1) pattern of the P t ( l l l ) polycrystalline ribbon at room tem perature. NO was found to reverse the P t(llO ) - (1x2) reconstruction resulting in a (1x1) pattern [43]. On the stepped P t [ 1 2 ( ll l) X (111)] a (2x2) overlayer structure form ed in which the half order diffraction features were split in the same direction as the prim ary diffraction features. This suggests that the adsorbed layer is well ordered on the terraces and that the step periodicity also occurs in the overlayer [44]. No ordered LEED pattern was observed in this work.
NO was also believed to adsorb on bridge and atop sites on P t ( l l l ) with the occupation sequence reversed com pared with that o f CO [39]. The vibrational band at ~ 1476cm"' observed at low coverages in the EELS spectra o f G land and Sexton [42] was assigned to bridge bonded NO (following com parisons with nitrosyl vibrational frequencies). This band com pletely converts to a band at 1700cm ‘ within a very narrow coverage region (0.12 < 0 < 0.16 where Osat = 0.25) which is assigned to atop NO. IRAS studies by Hayden [45] supported this conversion but suggested there were still some bridge bonded species present at saturation. The latter author also showed that the conversion occurs in reverse when the crystal is heated and desorption takes place. In recent years, however, the assignm ent of the saturation coverage band to atop NO has been called into question. LEED-IV data obtained by M aterer et al. [46] could only be fitted with NO adsorbed on 3-fold fee hollow sites. No low coverage experim ents were carried out in this study. D FT calculations by Ge and King [47] also found the fee hollow site to be the m ost stable at 0.25M L. The atop site was the least stable. The latter study proposed an upright configuration for NO on P t ( l l l ) with no tilting at saturation coverage.