Bending vibration of NO on Pt(111) at the intermediate-excited state in photostimulated desorption

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 352/353 (1995) 519-523

Bending vibration of NO on Pt(111) at the intermediate-excited state in photostimulated desorption Yoshitada Murata*, Katsuyuki Fukutani Institute ]br Solid State Physics, The University of Tokyo 7-22-1 Roppongi. Minato-ku, Tokyo 106. Japan

Received 15 October, 1994

Abstract

Laser-induced desorption of NO from Pt(111) at 80 K is studied using a nanosecond-pulsed laser at 193 nm. The desorbed NO molecules are state-selectively detected by the resonance-enhanced multiphoton ionization method. The tilt angle at the time of NO desorption is estimated from the rotational quantum number dependence of the time-of-flight spectra for the desorbed molecules using a simple impulse model. The potential surface for the frustrated rotation (bending vibration) of NO chemisorbed on the Pt(111) surface is found to be nearly flat at the intermediate-excited state from an analysis of the mean vibrational amplitude.

I. Introduction

Our interest in fundamental studies of surface science is now shifting from the determination of static properties, such as surface structure, electronic structure in the ground state and adsorption energy, to the elucidation of dynamical phenomena from quantum-mechanical aspects. One of the typical problems belonging to this category is the photostimulated desorption induced by valenceelectron excitation with quantum-state observation. The desorption induced by a nanosecondpulsed laser is a simple non-thermal process and is an elementary or nearly elementary process of surface reactions. We have studied N O and CO desorption from Pt surfaces induced by visible and ultraviolet nanosecond-pulsed lasers [1-4]. The desorbed neutral N O molecules are stateDedicated to Professor Yonezo Morino on his 87th birthday. * Corresponding author.

selectively detected by the (1 + 1)-resonanceenhanced multiphoton ionization (REMPI) method, by which the vibrational, rotational and translational energy distributions of desorbed N O in the ground electronic state are measured in detail. The desorption processes of N O and CO from Pt(111) and Pt(001) surfaces are considered to be described by a model of substrate-media~ed excitation proposed by G a d z u k et al. [5], Electronic excitation due to photon absorption occurs near the Pt surface and a hot electron is transferred to the 27r antibonding unoccupied state of adsorbed N O or CO, that is, a negative ionic state of the adsorbate is generated as an intermediate-excited state. When the residence time in the excited state of the negative ionic state is longer than a critical value, to the adsorbed molecule is desorbed as a neutral molecule at the time of deexcitation to the neutral ground state. However, when the residence time is shorter than tc, the molecule is recaptured,

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Y. Murata, K. Fukutani/Journal of Molecular Structure 352/353 (1995) 519-523

that is, desorption does not occur [6,7]. Therefore, it is important to know the critical residence time in the intermediate-excited state. Recently, we have found that the rotational energy distribution observed by the R E M P I method gives us useful knowledge on the critical residence time and the lifetime of the intermediate-excited state [7]. In order to estimate the lifetime and the critical residence time, the potential surface for the bending motion (frustrated rotation) of the adsorbate at the intermediate-excited state is indispensable. In this paper, we discuss the frustrated rotation of N O chemisorbed on the P t ( l l l ) surface at the intermediate-excited state in the desorption process.

2. Experimental and results The sample surface of Pt(111) was cleaned in an ultrahigh vacuum chamber. N O molecules were adsorbed at 80K. Desorption occurs at high coverages with visible or ultraviolet (UV) laser irradiation, while dissociation occurs at low coverages with UV laser irradiation [8]. An A r F excimer laser, of which the photon energy and the wavelength are 6.4eV and 193nm, respectively, was used to irradiate the sample at 80 K in the present study. The duration of the laser pulse was 11 ns and the repetition rate was 10 Hz. Since the laser fluence used in this experiment was about 1 m J cm -2, the surface temperature rise due to the laser irradiation was lower than 5 K. The desorbed neutral N O molecules were state-selectively detected by (1 + 1)-REMPI via the A2II (v'---0, J ' ) *- X2y]q/2,3/2 ('ott = O , J " ) transition using a frequency-doubled dye laser with a tuned wavelength region from 225 to 227 nm. The time-offlight (TOF) spectrum was measured by varying the delay time from p u m p laser irradiation to probe laser firing with a constant flight length. The molecules desorbed in the normal direction to the surface were detected. In the observation of the T O F spectrum, the probe laser wavelength was tuned for detected desorbed molecules with a rotational q u a n t u m number of J ' . Details of the experimental apparatus have been described elsewhere [4].

t-

O

z

,~ .~~ (d)J=30.5 0 10 20 Timeofflight(/1s) Fig. 1. Rotational quantum number (J) dependence of the TOF spectrum of NO (v = 0,1] = 1/2) from Pt(lll) with an ArF pump laser: (a) (O), J = 0.5-4.5; (b) (D), J = 15.5; (c) (II), J = 22.5; (d) (©), J = 30.5. The solid curves are the fit to the modified-Maxwellianform. Fig. 1 shows the J dependence of the T O F spectra of N O from P t ( l l l ) at 80K. The peak position shifts to a shorter flight time with increasing rotational quantum number. The result shows that the averaged translational energy increases with increasing rotational energy of desorbed molecules. Similar results were observed in other systems [9-12]. The averaged translational energy (Et) is obtained by fitting the T O F spectrum with the modified Boltzmann distribution, A v 3 e x p ( - a v 2 - by), as listed in Table 1.

3. Discussion We propose a simple impulse model describing Table 1 Rotational-quantum number (J) dependence of the mean translational energy ((Et)) and the tilt angle (0) at the time of desorption for NO from Pt(lll) at 193 nm J

(E,) (K)

Er (K)

V~r (K 1/2)

0.5-4.5 16.5 22.5 30.5

800 1580 1830 3420

1.8-60.4 1.3-7.8 705 26.5 1290 35.9 2340 48.4

$ (deg) 1.1-6.9 16.9 21.5 21.2

Y. Murata, K. Fukutani/Journal of Molecular Structure 352/353 (1995) 519-523

the J dependence of the T O F spectrum as well as the rotational excitation of the desorbed molecules [6,7]. The translational temperatures are much higher than the surface temperature, as seen in Table 1. The rotational temperature obtained from the (1 + 1)-REMPI spectra is also much higher than the surface temperature [2]. These results show that the desorption is induced by the electronic transition and is a non-thermal process. The excess energy Ek surpassing the potential barrier for desorption, i.e. the dissociation barrier of the metal-adsorbate bond, is given to the desorbed molecule. In the impulse model, the energy Ek and the momentum P0 are assumed to be given to only the N atom of desorbed N O bound to the substrate at the first step. The validity of this model arises from the time mismatch between the rotational and translational motions. That is,

Ek = p~/2ml

(1)

where mr is the mass of the N atom. The recoil of the substrate atom can be ignored, because the mass of a Pt atom is much larger than that of an N atom. Since the desorbed N O molecules sit on the on-top site on P t ( l l 1) [8], the momentum P0 corresponding to Ek is given in the direction parallel to the P t - N bond at the time of m e t a l adsorbate bond breaking. The energy Ek depends on the residence time in the excited state and is nearly zero when the desorption occurs just after to. Therefore, the momentum P0 depends on the potential energy surfaces in the desorption process and the F r a n c k - C o n d o n overlap in the excitation process as well as the residence time at the excited state. After the desorption, the coordinate system is changed to the center-of-mass system. Therefore, the momentum P0 is converted to the linear momentum for the center of mass P = P0, and the linear momentum for the internal coordinate p = (m2/M)p o is generated, where M and m2 are the mass of the N O molecule and the O atom, respectively. The energy Ek is partitioned by the translational (Et), rotational (Er), and vibrational (Ev) energies of the desorbed N O molecule,

Ek = Et + Er + Ev = P2/ZM + L2/2I + p2r/2# (2)

521

where I and # are the moment of inertia and the reduced mass of the N O molecule, respectively, the rotational angular momentum L =r ×p = pr sin 0, and Pr = P cos 0. Here, the angle 0 is the tilt angle of the NO molecular axis from the P0 direction at the time of metal-adsorbate bond breaking. Et and Er are readily calculated as follows:

Et = p2/2M

(3a)

E r = ( m 2 / M ) 2 p 2 sin 2 0/2]~

(3b)

Furthermore, the relation of

Er = (m2/m~)Et sin 2 0

(4)

is derived by eliminating P0 from Eqs. (3a)and (3b). The important part is that Eq. (4) is independent of Po, which is a complex product from the potential energy surface and the residence time at the excited state. The bending vibration of the adsorbate on the surface of the intermediate-excited state is determined by the J dependence of the T O F spectra shown in Fig. 1. The mean translational energy (Et) is given by a fit to a modified Maxwellian distribution, as listed in Table 1. The rotational energy is calculated from E r = B J ( J + 1), where the rotational constant B is equivalent to 2.44 K for the NO molecule. Substituting Er and (Et) in Eq. (3), the tilt angle 0 at the time of NO desorption can be determined as a function of X/~r- The square root of the rotational energy v/-E~ris proportional to the residence time at the excited state [6,7]. The results are listed in Table 1 and are plotted in Fig. 2. The tilt angle saturates at about 55 ° and reaches the turning point. This result indicates that the amplitude of the bending motion at the excited state is much wider than that in the ground state, 3.7 ° as described below. The mean amplitude of molecular vibration is calculated from the vibrational frequency [13,14] and vice versa. The mean square amplitude for the normal coordinate with an angular frequency ~ at an equilibrium temperature T is given by (Q2)r = (h/2~) coth(h~/2kBT)

(5)

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Y. Murata, K. Fukutani/Journal of Molecular Structure 352/353 (1995) 519-523

single-photon process, since the v' = 0 ~ v" = 0 transition is dominant if the potential energy surfaces of the ground and excited states have a similar minimum point. In such a case, the vibrational frequency can be estimated from

60 I d, "O

Q

4£

N 20

0

We = h/r212m2 (A02) I

210 ~ 4~r(K 1/2)

410

Fig. 2. Tilt angle at the time of desorption as a function of v~r, which is proportional to the residence time in the excited state, for NO from Pt(111) with an ArF pump laser.

which is derived from the virial theorem of ( A / 2 ) ( Q 2 ) v = ( h ~ / 2 ) ( v + l / 2 ) and A = w 2. The internal coordinates R are transformed by the L matrix from the normal coordinates Q. The present case is approximated by an internal coordinate of the bending angle Ri = AO = L q Q j , where Qj is the normal coordinate corresponding to the bending vibration of adsorbed NO. For the bending vibration of a linearly adsorbed molecule on the on-top site of a heavy substrate atom, ( A 0 2) = L i2j ( Q )2) = G j j ( Q 2) = ( Q )2) / r l 2z m 2

(6)

where Gjj is the corresponding element of Wilson's G matrix [15] and r12 is the N - O distance. Since the bending vibration of this system is doubly degenerate, it follows that (AO2) r = 2Gjj (Q2) = (h/r22m2 w) c o t h ( h w / Z k B T)

(7) The root-mean-square amplitude ~ at 80 K is calculated to be 3.7 ° from the bending wavenumber of ~ = o~/27rc = 380cm -1 in the ground state observed by high-resolution electron energy loss spectroscopy [16]. The bending frequency at the excited state Wecan be estimated from the vibrational amplitude if the vibrational-state population is in thermal equilibrium. However, the present case is not in thermal equilibrium. The vibrational excitation in the bending motion of adsorbed NO can scarcely occur during the electronic excitation via the

(8)

in which the effective amplitude is derived from the tilt angle at the turning point. The estimated value is very low; ~e = ~Oe/27rc ..~ 3cm -1. If the potential surface in the excited state has a different minimum position from that in the ground state, the vibrational excitation of a few quanta may occur. In conclusion, the potential energy surface of the frustrated rotation of chemisorbed NO in the excited state is nearly flat. This information should be important for discussion of the desorption process in photostimulated desorption.

Acknowledgment The authors thank Professor T. Iijima for valuable discussions.

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[15] E.B. Wilson, Jr., J.C. Decius and P.C. Cross, Molecular Vibrations, The Theory of Infrared and Raman Vibrational Spectra, McGraw-Hill, New York, 1955. [16] G. Pirug, H.P. Bonzel, H. Hopster and H. Ibach, J. Chem. Phys., 71 (1979) 593.