# Presentation: 716414

## Femtochemistry

**Philip Anfinrud, ***National Institutes of Health*
**John Arthur, ***Stanford Synchrotron Radiation Laboratory*
**Jerry Hastings, ***Brookhaven National Laboratory *
**Chi-Chang Kao, ***Brookhaven National Laboratory*** **
**Richard Neutze, ***Uppsala University, Sweden*
**Mark Renner, ***Brookhaven National Laboratory*
**Wilson-Squire Group, ***University of California at San Diego*
**Ahmed Zewail, ***California Institute of Technology*

***Femtochemistry***

## Description of static molecular properties in terms of bond lengths and angles has served us well.

**Description of static molecular properties in terms of bond lengths and angles has served us well.**
**Virtually every new discovery in biology and chemistry can be traced to a structure being solved.**

## Chemical transformations are about dynamics, i.e. rapid changes in bond lengths and bond angles.

**Chemical transformations are about dynamics, i.e. rapid changes in bond lengths and bond angles.**
**What is needed is a tool that will make possible a simple connection between the static picture and its time evolution.**

## The ultimate goal of any molecular dynamics study is to produce a motion picture of the nuclear motions as a function of time. 

**The ultimate goal of any molecular dynamics study is to produce a motion picture of the nuclear motions as a function of time. **

## BUT

**Femtosecond lasers are fast enough **
**BUT**
**Their greater than 200-nm wavelength does not allow for any spatial information**

**Capturing molecules *****in the process***** of reacting has been a **
**long-time dream**

## Spectroscopy of the transition state is an attempt to compensate for the inability of lasers to provide the spatially needed resolution

**Spectroscopy of the transition state is an attempt to compensate for the inability of lasers to provide the spatially needed resolution**

**Ultrafast Electron Diffraction (UED) is the only experimental system that attempts to break that limit **

## Putting things in perspective

**Putting things in perspective**
**What are the time-scales?**
**What are the length-scales?**

## Time in femtoseconds, distance in Å

**Time in femtoseconds, distance in ****Å**

**H****2****O********OH + H**

**CH****2****I****2********CH****2****I**** + I**

## TIME

**TIME**
**The very light systems require a time resolution of a few femtoseconds, while heavier ones can be studied with pulses a few hundred femtosecond long. **
**BOND LENGTH**
**The *****LCLS*****   will make it possible to map out the nuclear motions with a resolution of 0.1 Å, which is clearly sufficient. **

## Ultrafast Electron Diffraction

**Ultrafast Electron Diffraction**
**H. Zewail**

## UED will never break the psec time limit because of the fundamental relationship between the number of electrons in the bunch and pulse length.

**UED will never break the psec time limit because of the fundamental relationship between the number of electrons in the bunch and pulse length.**

**The *****LCLS****** *****is the only tool with the required temporal and spatial resolution **

## Comparison between Ultrafast Electron Diffraction (UED) and the LCLS

**Comparison between Ultrafast Electron Diffraction (UED) and the *****LCLS***

**1**** time resolution; ****2 ****relative crossection; ****3**** relative signals**

**The predicted signals are comparable but the *****LCLS****** *****time resolution is at least 50 times better.**

## Exp 1.     Gas phase photochemistry

**Exp 1.     Gas phase photochemistry**
**Exp 2.     Condensed phase photochemistry**
**Exp 3.     Dynamics in nanoparticles**

## The femtochemistry experiments use an ultrafast laser to initiate the process and the LCLS beam as a probe

**The femtochemistry experiments use an ultrafast laser to initiate the process and the *****LCLS***** beam as a probe**

##    Time resolved diffraction

**   ****Time resolved diffraction**
**   ****Time resolved Mie scattering (small angle scattering)**

## Photodissociation of an isolated diatomic molecule is the simplest of chemical reactions.

**Photodissociation of an isolated diatomic molecule is the simplest of chemical reactions.**
**   ****	****t=0 is easily defined **
** ****	****The initial wave-function is well**
**    ****defined**
**  ****	****The wave-function remains localized **
**    ****throughout the reaction**
**  ****	****The *****LCLS****** *****is ideally suited to **
**    ****investigate these reactions**

***    ******Experiment 1.  Gas phase photodissociation reactions***

## The coupling between nuclear and electronic motion is a universal phenomenon that dominates almost all photochemistry.  

**The coupling between nuclear and electronic motion is a universal phenomenon that dominates almost all photochemistry.  **
**   **
**It is essential that we develop an intuitive picture of this behavior**
***LCLS****** *****will make it  possible to directly observe this complex motion. **

***    ******Nuclear and Electronic Coupling is Universal Phenomenon***

## The solvent cage changes the dynamics and provides a means to study recombination reactions.  

**The solvent cage changes the dynamics and provides a means to study recombination reactions.  **

***    ******Experiment 2. Condensed phase photochemistry***

## I2 in dichloromethane (Neutze et al.)

**I****2**** in dichloromethane (Neutze *****et al*****.)**

***    ******Third Generation Sources Have a Limited Time Resolution***

**Diffuse X-ray scattering with**
**80 psec time **
**resolution from**
**European Synchrotron Radiation Facility**

## I2 in dichloromethane (Neutze et al.)

***    ******An Example from E S R F***

**I****2**** in dichloromethane (Neutze *****et al*****.)**

## Nanoparticles

**Nanoparticles**
**Semiconductors and metal nanocrystals also known as quantum dots possess unique size-dependent electronic and optical properties that result from quantum size confinement of charge carriers and very large surface to volume ratios.**
**These properties hold great promise for applications in areas such as microelectronics, electro-optics, photocatalysis, and photoelectrochemistry.  They are also particularly attractive, because of their large surface area and fast charge transport properties, for photovoltaics and photo-degradation of chemical wastes and pollutants. **

## The size distribution problem

**The size distribution problem**
**Under most experimental conditions size dependent properties tend to be masked by the presence of a wide size distribution. The high intensity of the *****LCLS****** *****will make it possible to conduct experiments on *****single particles*****. **
**The solvent effect**
**Under most experimental conditions the high surface to volume ratio results in extreme sensitivity to solvent. To provide for a controlled, ****reproducible, well defined, ****inert environment, with low scattering background particles will be isolated in Ne crystals for study. **

## Melting a single nanoparticle

**    *****Experiment***** *****3. Melting single nanoparticles***

## The time evolution of the Mie scattering spectrum at 1.5 Å will make it possible to map out internal particle vibrational modes as well as surface capillary modes of a single nanoparticle. 

**The time evolution of the Mie scattering spectrum at 1.5 Å will make it possible to map out internal particle vibrational modes as well as surface capillary modes of a single nanoparticle. **

##     Simulation of Mie Scattering at 1.5 Å

***    ******Simulation of Mie Scattering at 1.5 Å***

**Simulated scattering intensity at a single angle as a function of particle size. A similarly rich spectrum is obtained for a fixed particle size as a function of scattering angle. **
**Mie spectra are extremely sensitive to changes in particle size and shape. **

## The LCLS is the only tool that will, in the foreseeable future, make it possible to observe nuclear motion during a reaction in real time.

**The *****LCLS***** is the only tool that will, in the ****foreseeable**** future, make it possible to observe nuclear motion during a reaction in real time.**
**The *****LCLS***** can be applied to a wide range of problems in the field of chemistry, some of which were touched upon here, from the most fundamental photodissociation reaction, to the more applied problem of characterizing the properties of nanoparticles. **