Hot STM Labs

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 Rosch1 is 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.

Hot STM Labs

Copyright 1998 by the Regents of the University of Minnesota, Dept. of Physics & Astronomy. All rights reserved.