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Figure 1: A schematic for a combined femtosecond laser system with scanning tunneling microscopy (fs-STM). Edited from [2]
Figure 2: An illumination scheme for photo-injected electrons from metal surface into the ammonia clusters.
Figure 3: An optical ring with two parabolic mirrors couples the laser beam with the tunneling region of STM. [2]
Figure 4: A schematics showing the illumination of tunneling region of STM using a laser pulse guided by the parabolic mirrors of th optical ring.
Figure 5: Illumination of a two-layered ammonia cluster via ultrashort pulses (36 fs) leading to diffusion and formation of a bigger cluster.

Fs-STM: A unique method to study the effect of electron solvation on molecular scale

Why study electron solvation?

Electron solvation in polar solvents like water/ ammonia plays a key role in various fields like atmospheric sciences. Electron trapping in the ice structures of these solvents enhances reduction of CFC-12 thereby playing a key role in ozone layer depletion [1]. Thus, there is a fundamental need to decipher the dynamics and molecular scale insight of solvation.

What is fs-STM?

Combining a femtosecond (fs) laser system with a scanning tunneling microscope (STM) opens a unique possibility of tracking the reactions induced by ultrashort pulses on the molecular scale. We named this technique: fs-STM (figure 1).

This system constitutes a molecule-metal interface. Ammonia/water solvent molecules adsorb on a metal surface and form clusters. Since the bandgap of a molecular cluster is large and non-resonant with the energy of laser pulse, direct excitation of the structure does not happen. The femtosecond laser pulses are absorbed in the metal surface, and the low-energy electrons from the surface inject into solvent structures, which then solvate (figure 2).

One of the advantages of this technique is that we inject photo-excited electrons into the solvent structures. In liquid ammonia, not only the electrons from the alkali metals are solvated, but the alkali cations also get solvated. In the metal-molecule system, only electrons from the surface reach the solvent structure while the photo holes get screened. Thus we study the effects of solvated electrons on the solvent structures.

This technique requires an understanding of two different machines, i.e., femtosecond laser system and ultrahigh vacuum low-temperature STM. Since the start of my Ph.D. in May 2018, I took six months to understand this combined technique.

How do we know that a reaction is induced by low energy electrons excited by ultrashort pulses?

Our low-temperature STM provides a possibility where the temperature reaches down to ultra-cold and all molecular motions cease to occur. Thus, a reaction can only occur when we excite the system.

What is the challenge?

The main challenge of this system is to study the reaction at the same spot that is excited by ultrashort laser pulses, having an area in the range of tens of micrometers, and the STM tip can scan an area that is much smaller than the spot size.

For this, we use an optical ring holding two parabolic mirrors (figure 3), which allows us to focus the laser spot under the tip. Thus we can image the same spot before and after the laser excitation. The metallic sample is at the focal length of these two mirrors, and the laser beam focuses on the surface. The alignment is done with the help of two pilot lasers, i.e., green and red. One laser serves as a background while the other laser is aligned under the tip by moving the optical ring that stands on three piezos (Figure 4). The ultrashort laser pulse is aligned with this red laser pulse outside the STM, and thus the laser pulse hits the spot where the tip is. In this way, we can scan the changes before and after the excitation (Figure 5).

How can we overcome shortcomings?

A shortcoming of this technique is the simultaneous illumination of the laser and scanning with the STM tip. We cannot observe phenomena occurring on femtosecond/picosecond timescale and the changes that occur in this time scale.

In order to overcome this shortcoming, we will introduce a beam splitter in our beam path to split the laser into two equal parts and use a delay stage to make the delays on femtosecond timescale between the two laser pulses. This delay stage can make the beam path of one laser pulse longer with nanometer precision on the timescales of femtosecond with respect to the other laser pulse. By varying the delay from the time when both pulses don’t overlap each other, we move our delay stage to reach zero delay between the two pulses. At every delay step, we illuminate the metal surface inside STM with a laser and image the changes. In this way, we might be able to sketch the changes at each step and elucidate the changes on the femtosecond timescale.

Obtaining molecular-scale insights into the solvation of electrons in solvent structures is a bottom-up approach of studying the electron-solvent interaction which paves a way of our understanding on a larger scale in the mundane world.


[1] Q.-B. Lu and T. E. Madey , Giant enhancement of electron-induced dissociation of chlorofluorocarbons coadsorbed with water or ammonia ices: Implications for atmospheric ozone depletion J. Chem. Phys., 1999, 111 , 2861

[2] Mehlhorn M., Gawronski H., Nedelmann L., Grujic A. & Morgenstern K. An instrument to investigate femtochemistry on metal surfaces in real-space. Rev. Sci. Instrum. 78, 033905


About the author

Prashant Srivastava is a PhD student under the supervision of Prof. Dr. Karina Morgenstern. In his master thesis, he has studied the magneto-optical properties off rare earth elements using ultrafast pump-probe spectroscopy. In his PhD project, he investigated the electron solvation in real space on ammonia solvent using scanning tunneling microscopy and ultrafast laser system. He is currently working on achieving time resolution of electron solvation using time delay experiments.