# Presentation: 070488

## Atomic Physics with Intense X-rays at LCLS 

**Philip H. Bucksbaum, ***University of Michigan, Ann Arbor, MI*
**Roger Falcone, ***University of California, Berkeley, CA*
**Richard R. Freeman, ***University of California, Davis, CA** *
**Kenneth Kulander, ***LLNL, Livermore, CA*
**Linda Young, ***Argonne National Laboratory, Argonne, IL*

## Dual Motivation to Perform Atomic Physics Studies

**Fundamental Science**
**The LCLS, as a high-intensity high-energy photon source,**** provides a unique opportunity to study fundamental aspects**** of x-rays interacting with atoms, ions, molecules, and clusters**
**Foundation for all experimental planning**
**The understanding of x-ray–atomic physics interactions is ****central to experimental designs at the LCLS, as well as all next****generation x-ray sources.**

## The LCLS will Reach Regime that are Currently Unobtainable 

**• ****Current laser-atom process at ****I**** ≥ 10****14**** W/cm****2**

**amplitude of ****free e****-**

**ion core**

**• ****LCLS-atom process at ****I**** ≥ 10****14**** W/cm****2**

**• ****Field modulates the atomic ****potential at visible laser frequency**
**• ****Outer e****-**** has time to tunnel free:**
**	****     • ****2U****p**** > I****p****   where U****p ********I**** ********2****)****2**
**• ****Strong interaction between free e****-**** and ion core is of interest**

**• ****Field modulates the atomic potential at x-ray laser frequency**
**• ****e****-**** do not have time to tunnel free**
**• ****Important processes are with deeply bound core e****-**

## New Fundamental Processes will be Observable

**Experiment 1: Multiple ionization sufficiently rapid to form hollow atoms**

**Experiment 2: Multiphoton ionization yielding absorption below the edge**

**Experiment 3: Giant Coulomb explosions of clusters**

**Notes:**
All LCLS applications involve XFEL-atom interactions, so understanding basic atomic physics is one of the most important early tasks. 
The intensity and wavelength regime of LCLS place it apart from third generation sources, and some of the x-ray science will be different. 
We propose investigating this in controlled experiments, using the range of focus conditions and wavelengths available at LCLS.

## The Experimental Setup for all these Experiments is the Same

**source**

-  **Detectors**
** ****Charge state spectrometer**
** ****Electron energy spectrometer**
** ****Ion recoil detector**
** ****X-ray fluorescence detector**

***The Experimental Setup for all these Experiments is the Same***

**Charged**
**particle**
**detector**

**• ****Tunable LCLS**

**X-ray**
**detector**

**Notes:**
Add x-ray detector

## Experiment 1: Multiple ionization forming hollow atoms

**Ionization: ********= 10****12****s****-1**
**Auger: ********= 4x10****14****s****-1**

**75 events/pulse**

**Notes:**
Unfocused beam

## Multiple Ionization Forms Hollow Atoms

**Neon will display the effect well**
**Relatively high photoionizations**
**Non-corrosive monatomic sample**
**Simple, well understood spectrum**
**Relatively long Auger decay rate (2.5 fs)**
**Auger relaxation > 100 x radiative fluorescence**

- n=2

- n=1

**Ne Photoionization**

**Notes:**
Neon shows

## Possible Ionization Processes for LCLS Interacting with Ne

**Photoionization: **
**	****Ne + h********>870eV**** ******** Ne****+*****(K) + e****	**
**Auger Decay: **
**	****Ne + h********>870eV**** ******** Ne****+*****(K) + e ****	******** Ne****2+*****(LL) + e**
**	****                        ****	****	****	****	******** ****Ne****3+***** + 2e**
** **

**LCLS only**

**  ****Sequential multiphoton ionization:**
**	****Ne + h********>870eV**** ******** Ne****+*****(K) + e + h********>993eV**** ******** Ne****2+*****(KK) + e ****	******** Ne****3+**** + e**
**	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	******** ****Ne****4+**** + 2e **
**	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	******** ****Ne****5+**** + 3e**
**	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	****	******** ...**
**	****Ne + h********>870eV**** ******** Ne****+*****(K) + e + h********>993eV**** ******** Ne****3+*****(KLL) + e**
** **
**  ****Direct multiphoton ionization:**
**	****Ne + 2 h********>932eV**** ******** Ne****2+*****(KK) + 2e****	**

## One Photon or Two? An Extremely Difficult Question for    Multielectron/multiphoton Systems

**The intensity of the LCLS makes numerous processes possible/probable**
**For example: (KL) double vacancies are possible:**** **
**	****	****Ne +**** ****h********>910ev**** ******** Ne****2+*****(KL) + 2e (10%)**
**	****	****	******** ****Ne****3+**** + e (9.5%)****	**
**	****	****	******** ****Ne****4+**** + e (0.5%)**
**Experimentally background signals of this type can be rejected by electron spectroscopy**
**Calculationally simulations of the LCLS atom interactions and the core relaxations are necessary**

***• ******LCLS will allow the study of detailed multiphoton atomic core processes***

## Experiment 2: Focused beam experiments

**Kirkpatrick-Baez mirror pairs (demagnification factor of ≈ 100)**

**Saturation: photoionization rate *****equals***** the Auger decay rate**

**LCLS**

**Kr source**

**detector**

**• ****Focusing permits observation of two-photon photoabsorption**

**2x10****6**** events/pulse**

## 2-Photon Absorption in Kr

- n=2

- n=3

**Kr photoabsorption**

- n = 3

- n = 1

- n = 2

- n = 4

- 1700 ev

**Kr energy levels**

**2 LCLS photons with h********>850eV**

**Schematic of Kr ionization process**

*h*

- 2*h*

**Notes:**
Add arrows on energy axis

## 2-Photon Absorption – Detecting Events

** ****Kr +**** ****2h********>850eV**** ******** Kr****+*****(L) + e****	**

**• ****Kr****+*****(L) + e**** ******** ****Kr****+*****(M)+**** ****h********1.5keV**** **

**• ****Kr****+*****(L) + e ******** Kr****+*****(MM) + e(1.5keV****)**

**• ****Detection signatures:** radiation *and*** **2x106 1.5 keV e-/pulse

**• ****Excitation mechanism**

**radiation**

**particles**

## Theory of Resonant 2-Photon Processes Requires Data   Only LCLS Can Provide

- Huge enhancements associated with single photon resonances S. A. Novikov, J. Phys. B. 33 (2000)
- 2-photon rate exceeds 1-photon rate!
- Rate can be affected by coherence and enhancement due to correlation

- •   	2-photon ionization couples to an intermediate state
- •  	1s22s22p6
-    	1s2s22p6**n**p
-    	1s2s22p6+e-

**Notes:**
Middle line should be sum over lots of states

Here are my additional thoughts on inner shell multiphoton ionization.
They stem from the following three papers.
        1) S. A. Novikov & A. N. Hopersky, J. Phys. B. 33 (2000), 2287-2294.
           "Two-photon excitation/ionization of atomic inner shells"
        2) J. Abdallah Jr., et al, Z. Phys. D 34, 233-239 (1995).
           "Two-photon ionization of an inner shell electron of the Cl atom"
        3) W. Zernik et al, Phys. Rev. 135,A51 (1964)
           "Two-photon ionization of atomic hydrogen"

1. So far, we've only considered non-resonant inner-shell MPI. (e.g. 1s -> 2p, 31 au; 2s -> cont, 1.8 au; 2p -> cont, 0.73 au)
followed by absorption to the continuum!  These are some 14 orders of 
magnitude greater than the non-resonant two photon excitations, i.e. around
10^-42 cm4s.  These resonances are analogous to your ATI work- but resonant.
We could also see these two-photon resonances somewhat "cleanly" by looking at 
the photoelectron spectra for electrons of the proper kinetic energy, roughly
the photon energy.  These structures could be interesting/important as the
rate of two photon absorption may exceed that of single photon absorption.

Let's take the case of Ne and the two photon absorption resonance 1s->2p->cont.
at 31 au (849 eV) with a two-photon absorption cross section of 10^-38 cm^4s!
Then the two photon rate is 10^-38 cm^4s *(10^31 ph/s/cm2)^2 = 10^24 /s, and
within the pulse duration the atom is sure to be 2-photon ionized.  For
the single photon rate, the cross section is 2.4 x 10^-20 cm^2, and the
intensity is still 10^31, giving 2.4 x 10^11 /s.  So we would get more
two photon events than single photon events.  A critical intensity could
be defined:
        Two photon rate = one photon rate
        sigma (2ph) * Icrit^2 = sigma (1ph) * Icrit

  or    Icrit = sigma (1ph)/sigma (2ph).

A signature of the two photon events would be photoelectrons of the proper
energy (around 850 eV) and of the single photon events would be Augers at
around 805 eV.  So it looks like we can monitor the ratio of the 2ph to
1ph events by photoelectron spectroscopy.  This may be of interest as a
bridge to higher order nonlinear effects.

2. The two photon resonances 1s -> 3s,3d (which do not have a nearly
resonant intermediate state) are much less pronounced than
the other resonances mentioned above.  Only rises 2 orders above the
background (around 10^-53 cm^4s).

3. There are similar resonances calculated in Zernik's paper on metastable
hydrogen (2s) - where the singularities are at the Balmer lines.

4. I tried to "check" the estimate of the two photon cross section of
Kr L-shell using the scaling laws of Zernik and came up with the following.

        Sigma(2ph)[E,Z] = Z^-8 Sigma(2ph)[E/Z^2, 1].

Z^-8 comes from:        energies go as Z^2.  
                        dipole matrix from g->interdiate state goes as Z^-1
                        dipole matrix from intermediate ->final goes as Z^-5/2.

So for Kr 2p3/2 at 850 eV:
        E/Z^2 = 0.655 eV and no two photon ionization
        define Zeff by:  Zeff^2 * IP(H 2s) = IP (Kr 2p3/2) = 1678 eV
                         Zeff = 22.22

Then   Sigma(2ph)[1700eV,22.22] = Sigma(2ph)[3.44 eV,1] Zeff^-8
                                = 1.68 x 10^-11 * Sigma(2ph)[3600 Angstrom]
                        appx    = 1.43 x 10^-42 cm^4/W or 8 x 10^-61 cm^4s

this number is smaller than the rough estimate and too hard to explain, so
let's stick with rough estimate.  I include this here just for your interest.

5. As another point of reference, the Cl 1s two photon absorption rate 
has been calculated by Abdallah et al at 1.6 keV.  The K edge is at 2833 eV.  
So this is similar to a non-resonant two photon absorption into the
continuum.  Their value was 10^-41 cm^4/W, or 2.6x10^-57 cm^4s.  So the
estimate may be a little high.

To conclude - I think the only really new thing is that there may be interesting
resonant structure to probe (see Novikov's figures).  The neon resonant 
structure around 850 eV looks promising for these initial LCLS tests.  However,
the signature is not as clearcut for these huge resonances (which have
resonant intermediate states) as for the normal 1s -> ns, nd resonances -
since the photoelectron energies may be closely spaced.

I've not looked closely at your slides yet, but will get to it soon.

Linda

## Experiment 3: Intense X-ray beam interacting with clusters

** ****Detectors**
-  **Charge state spectrometer**
** ****Electron energy spectrometer**
** ****Ion recoil detector**

***Experiment 3: Intense X-ray beam interacting with clusters***

**detector**

**  ****LCLS **
**focused and unfocused**

## Cluster Explosion Experiment with Unfocused Beam

**Xe clusters (10****9**** atoms)**** **
**Each atom exposed to the unfocused beam will undergo:**
**~1 ionization event (****10****31 ****photons/cm****2****/s x10****-19 ****cm****2 ****x10****-13 ****s)**** **
** ********the ionization will saturate**** **
**The dominant relaxation mechanism is Auger decay**
**Therefore, each ionized atom creates 2 or more electrons**** **
**The cluster becomes a ball of charge with ~ 10****9 ****ions **
**Yields fast electrons, fast ions, and x-rays**

***•     ******Due to x-ray penetration the Coulomb explosion >> conventional lasers***

**Notes:**
Since ionization cross-sections for the heavier elements at 15 angstroms are on the order of 10-19 cm2 , during the pulse time of 200 fsec, That is,.  Now, each atom is being ionized from an inner shell, with, in the unfocused beam. (We point out here, that. This will represent a plasma of sufficient density that no optical photon could have penetrated to have produced it in the first place.  But since the LCLS is lasing at 15 Angstroms, the ball is easily penetrated and completely ionized.

## Cluster Explosion Experiment with Focused Beam

**Focusing the LCLS beam to 0.01 ********m**
**Each atom in the cluster will be classically-ionized nearly 10****4**** times over**
**The atom will continue to ionize, as the ~0.1 fs Auger rates ****are ~ 1000 times faster than the ionization rate**** **
**Thus, each atom will ionize until it strips down to**** the core level of the initial ionization event**
**Understanding these processes in detail ****is central to the imaging of bio-molecular ****samples**

- Lysozyme molecule
- irradiated by LCLS

## Summary:  Dual Payoff From Atom Studies

**Fundamental Science**
**LCLS is a unique opportunity to study new fundamental multiple ****photon x-ray  phenomena.**** **
**Foundation for all experimental planning**
**Ionization and cluster dynamics are central to experimental designs ****at the LCLS, as well as all next generation x-ray sources.**