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A Metastable Initial State in Benzene Adsorption

We have investigated the adsorption of benzene on the Si(001) surface using scanning tunneling microscopy. The central result is that benzene initially adsorbs in a metastable state, A, and then converts to a lower energy final state, B. The conversion to the final state occurs rather slowly at room temperature, with a time constant of 19 minutes. We study in detail the appearance of each state and its position relative to the underlying surface dimers in order to make a correspondence to existing proposed models for the structure of benzene adsorbed on Si(001). We measure a lower bound on the energy difference between the two states of 0.14 eV.

Experiment

(400 Å x 400 Å)

(400 Å x 400 Å)

About 8 minutes after depositing 0.06 L of benzene at room temperature, Figure 1, the benzene adsorbates are predominately in state A which appears bright and symmetrical. Nearly half an hour later, Figure 2, the benzene adsorbates have almost all converted to state B which is fainter and anti-symmetrical. Measuring the transition rate from state A to state B allows the determination of a barrier to the transition of 1.0 eV. After heating the surface to 350 K to hasten the approach to an equilibrium Boltzmann distribution of benzene molecules between the two states, less than one in a hundred benzene molecules remained in state A. This implies a minimum energy difference between the two states of 0.14 eV.

Comparison with Theory

Experimental Observations
(40 Å x 40 Å)
State A On top of a Si dimer Symmetric about dimer	State B Between two Si dimers Tilted: Adjacent bright and dark regions
In Figures 3 and 4, the locations of individual, buckled, substrate dimers are marked by white dashes. The black dash marking the center of the state A benzene atom in Fig 4 maps onto a substrate dimer. The separation between the light and dark halves of the state B benzene molecule in Fig 4 is equal to the separation of substrate dimers, (3.84 Å.)

Theoretical Models

	Correspondence to STM images
Pedestal Between dimers Symmetric	(not observed)	0 eV Birkenheuer, Gutdeutsch, and Rosch¹
Butterfly: Top of dimer Symmetric	State A	- 1.39 eV Birkenheuer, Gutdeutsch, and Rosch¹
Tilt Between dimers Tilted	State B	Proposed by Lopinski, Fortieer, Moffatt, and Wolkow²

The high energy Pedestal state proposed by Birkenheuer, Gutdeutsch, and Rosch¹is not observed, however the Butterfly state's symmetry and placement over a dimer suggest its identification as the observed state A. A good candidate for the observed state B is the Tilt state proposed by Lopinski, Fortieer, Moffatt, and Wolkow². The asymmetry and placement of the benzene in the Tilt state agrees with that of state B. Another appeal of the Tilt state is that a conversion from the Butterfly state is easily conceived. On a heated surface, the benzene molecule adsorbed in the Butterfly state could wobble, and bond to an adjacent dimer to form the Tilt state. While semi-empirical calculations have been performed for these and other proposed states, only the functional density calculations, as performed by Birkenheuer, Gutdeutsch, and Rosch and Lopinski¹ and Fortieer, Moffatt, and Wolkow³ find the unobserved pedestal configuration unstable.

¹Gutdeutsch, and Rosch and Lopinski. Surface Science.

²Lopinski, Fortieer, Moffatt, and Wolkow. JVST A.

³Wolkow, Lopinski, and Moffatt. Work in progress.

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