multiple photon absorption in hydrated cesium ion clusters jordan beck, jim lisy june 17,2008 osu...

Multiple Photon Absorption in Hydrated Cesium Ion Clusters

Jordan Beck, Jim LisyJune 17,2008

OSU International Symposium on Molecular Spectroscopy

Initial Motivation Project started with desire to find a unique IR

signature of magic number clusters similar to Johnson, Duncan, Jordan, et. al. with H+(H2O)n

Science 304 (5674), pp. 1137-1140, 2004

Initial Motivation There exists in the

literature clear evidence of magic numbers in hydrated alkali metal clusters

Example mass spectra from Castleman

We wanted to see if there was an IR signature to these magic number clusters

J. Phys. Chem., Vol 99, No. 19, 1995

Experimental Overview

Ion Gun

Conversion Dynode/ Electron Multiplier

Electrostatic Lenses

Conical Nozzle

Ion Deflector

Ion Selecting Quadrupole Mass Filter

Ion Guiding Quadrupole Ion Guide

Ion Analyzing Quadrupole Mass Filter

Source Chamber

Detection Chamber

Skimmer

Ion Guiding Octapole Ion Guide

Ion Guiding Chamber

Continuum Surelite II-10 Hz Nd3+:YAG (1064 nm)

Tunable LaserVision OPO/A

Ion Selecting

Quadrupole Mass Filter

Ion Guiding Quadrupole Ion Guide

Ion Analyzing

Quadrupole Mass Filter

Detection Chamber

Continuum Surelite II-10 Hz Nd3+:YAG (1064 nm)

Tunable LaserVision OPO/A

Experimental OverviewTypical Experiment

1) Select cluster ion of interest in first quadrupole (Q1), e.g. Cs+

(H2O)20

2) Laser interaction in second quadrupole (Q2)

3) Third quadrupole (Q3) set to monitor loss of most labile ligand, e.g. Cs+(H2O)19

The Solution

3) Third quadrupole (Q3) set to monitor multiple ligand loss channels, e.g. Cs+(H2O)17,18

First two steps are the same

The Problem

The loss of one water in the typical experiments leads to large background in large clusters due to spontaneous evaporation

Cs+(H2O)8

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Loss of 1 Water

An Example of a ‘Typical’ ExperimentCs+(H2O)8

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Loss of 1 Water Loss of 2 Waters

Cs+(H2O)8

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Loss of 1 Water Loss of 2 Waters Loss of 3 Waters

An Example of Monitoring Multiple Loss Channels

First Surprise: We are able to see the loss of three water molecules

Second Surprise: The spectra seem to change

Cs+(H2O)8 - Scaled to Free OH Peak

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Observations

1) Peaks get narrower as more waters lost.

2) Peak centers shift as more waters lost.

3) Some peaks get suppressed as more waters lost.

Multiple Loss Channels Show Different Features

We noticed that the cross sections of the loss channels decrease with the number of waters lost.

This begs the question: Do the intensities decrease uniformly over the whole spectral range?

One way to check is to scale the spectra so that they have the same free OH peak intensity.

Cs+(H2O)22 - Free OH

3660 3670 3680 3690 3700 3710 3720 3730 3740 3750 3760

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Loss of 2 Waters Loss of 3 Waters Loss of 4 Waters Loss of 5 Waters Loss of 6 Waters

Cs+(H2O)22 - Free OH - Scaled

3680 3685 3690 3695 3700 3705 3710 3715 3720

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Loss of 2 Waters Loss of 3 Waters Loss of 4 Waters Loss of 5 Waters Loss of 6 Waters

2

3

4

56

Looking at Free OH More Closely

Here we see loss of up to 6 waters!Also, a narrowing of the free OH peak with successive water loss.

Why and how do we see multiple ligand loss?

Initial InterpretationWater Binding Energy ~ 30-55 kJ/molArgon Binding Energy ~ 5 kJ/molPhoton Energy = 37-45 kJ/mol

Multiple Photon Absorption Necessary to Lose Multiple Waters

What are the mechanisms for Multiple Photon Absorption?

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quasi-continuum

hvxxxxxx

hvx

xxxxxxhv

x

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x

Coherent absorption: Probability of absorption of n photons proportional to nI

Can be ruled out due to anharmonicity of OH stretch and power dependence data.

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Heat Bath Vibrational Mode Levels

hvxxxxxxx

Energy leaks to

bath

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Repeat

Incoherent absorption: Probability of absorption of n photons proportional to I

Cannot be definitively ruled out.

Theor Chem Acc 114, 357-379 (2005)

Cs+(H2O)18 Cs+(H2O)8

J. Chem. Phys. 126, 074302 (2007)

Implications of Incoherent AbsorptionWe would expect the intensity of the hydrogen bonded features to grow relative to the free OH peaks with multiple water loss.

1. Intrinsic absorption cross sections of H-bonded modes larger than free OH modes

2. H-bonds are more effectively coupled into the framework of the molecule than the free OH modes

Cs+(H2O)n - Scaled

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Freq (cm-1)

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Loss of 1 Water Loss of 2 Waters

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Cs+(H2O)nAr - Scaled

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Loss of Ar Loss of Ar(H2O)

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Spectra Broken Down Into General RegionsFree Cyclic HB Bent HB

Sym & Asym

Linear HB

Free FreeSym Sym

Bent HB

Bent HB

Cyclic HB Cyclic HBLinear

HBLinear

HB

J. Chem. Phys. 126, 074302 (2007)

A closer look at Cs+(H2O)8

Eight identical free OH oscillators with identical absorption cross sections

Freq

(cm-1)Loss of

H2OLoss of (H2O)2

Loss of (H2O)3

Total Depletion σF P(0) P(1) P(2) P(3) P(4)

Cs+(H2O)4 3715 11.6 0.5 0.0 12.1 0.1 87.9 11.3 0.7 0.0 0.0

Cs+(H2O)5 3706 28.3 6.4 0.4 35.1 0.4 64.9 28.1 6.1 0.9 0.1

Cs+(H2O)6 3707 24.6 9.1 1.0 34.7 0.4 65.3 27.8 5.9 0.8 0.1

Cs+(H2O)8 3701 16.2 6.6 1.4 24.2 0.3 75.8 21.0 2.9 0.3 0.0

Cs+(H2O)10 3702 24.2 17.8 9.1 51.1 0.7 48.9 35.0 12.5 3.0 0.5

% Fragmentation Poisson Calculations

If the photon absorptions are random, uncorrelated events, then they can be modeled with a Poisson Distribution:

;

!

x FF eP x F

x

cross section

F Fluence

F 'Average' number of photons absorbed

number of photons absorbed

Total depletion

x

D

ln(1 )F D

Small non-argonated clusters dissociate one water molecule per photon (red box).

Multi-Photon Absorption

RRKM/EE formalism can be used to model these experiments

Rice-Ramsperger-Kassel-Marcus (RRKM) technique used to obtain rate data

Evaporative Ensemble (EE) technique used to model internal energy distributions.

0 20 40 60 80 100 120 140 160 180 200

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Energy (kJ/mol)

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Energy Distributions of CsW9-10

CsW10-Q2Entrance

CsW10-DetectorCsW9-Detector

Spontaneous Fragmentation: 5.1%

Experimental: 7.8%

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Energy (kJ/mol)

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Absorption of 1 Photon at 3700cm-1, 35% Absorpton

CsW10-MidQ2CsW10-Detector


Cluster Relative IntensityCs+(H2O)10

Cs+(H2O)9

Cs+(H2O)10

1.0 at Mid Q20.65 at Detector0.312 at Detector

Cs+(H2O)8 0.038 at Detector

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Energy (kJ/mol)

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Absorption of 1-3 Photons at 3700cm-1

CsW10-MidQ2CsW10-DetectorCsW9-DetectorCsW8-DetectorCsW7-Detector

Cluster Model Experiment # Photons % Clusters Absorbing9 27.7% 24.2% 1 35.0%8 13.4% 17.8% 2 12.5%7 3.9% 9.1% 3 3.0%

%Fragmentation

Conclusions

Clear, unambiguous evidence of multiple photon absorption

Different OH stretches exhibit different multiple photon behavior

Modeling experiments with RRKM/EE techniques and Poisson statistics is pointing us in the right direction

Acknowledgements

Professor Jim Lisy Lisy Lab

Jason Rodriguez Amy Nicely Oscar Rodriguez

Dr. Timothy Vaden Dr. P. Tarakeshwar Funding: NSF CHE 0415859

n Loss of 1 Loss of 2 Loss of 3 Loss of 44 2 1

5 5 2

6 6 3

8 20 15 20

10 8 12 7

19 20 19 23

20 29 26 15 18

21 34 27 29

H-bond / Free OH (Integrated Cross Sections)

Power Dependence

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Laser Power (mJ/pulse)

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Cs(H2O)6_-(H2O) Cs(H2O)6_-(H2O)2 Cs(H2O)6_-(H2O)3

Total Depletion

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TQ Entrance Power (mJ/pulse)

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Cs(H2O)4Ar Cs(H2O)5Ar Cs(H2O)4 Cs(H2O)5 Cs(H2O)6

Similar Features Observed in Larger Clusters

Cs+(H2O)19-21 Scaled

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Loss of 1 Loss of 2 Loss of 3 Loss of 4

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Cs+(H2O)19-21 Not Scaled

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Loss of 1 Loss of 2 Loss of 3 Loss of 4

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Freq (cm-1) Loss of H2O Loss of (H2O)2 Loss of (H2O)3 Loss of (H2O)4Total

DepletionCs(H2O)19 3696 35.3 21.7 6.1 - 63.1Cs(H2O)20 3696 29.1 25.2 5.7 1.2 61.2Cs(H2O)21 3696 29.9 13.4 6.7 - 50.0

σF P(0) P(1) P(2) P(3) P(4) P(5)Cs(H2O)19 1.0 37.0 36.8 18.3 6.1 1.5 0.3Cs(H2O)20 0.9 38.8 36.7 17.4 5.5 1.3 0.2Cs(H2O)21 0.7 50.0 34.7 12.0 2.8 0.5 0.1

Poisson Calculations

% Fragmentation

One Photon Causes Multiple Water Loss

Experimental fragmentation of one water loss less than calculated value.

Experimental fragmentation of two water loss greater than calculated value.

Notes: 1. This indicates that the absorption of one photon can cause the loss of more than one water.

2. There is still significant population in the loss of 3 (and 4) waters channels. This cannot be accounted for by one photon absorption.

Cs+(H2O)20 as an Example

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0.03No Photon Absorbed, BE=44kJ/mol

Energy (kJ/mol)

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CsW20-Initial


Model of Experiment – Free OH

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Energy (kJ/mol)

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Absorption of 1-4 Photons at 3700cm-1 in Poisson Distribution

CsW20-InitialCsW20-DetectorCsW19-DetectorCsW18-DetectorCsW17-Detector

Spontaneous Fragmentation Modeled

20 40 60 80 100 120 140 1600

0.005

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0.015

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0.03No Photon Absorbed, BE=44kJ/mol

Energy (kJ/mol)

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CsW20-Initial


Laser Induced Fragmentation Modeled

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Energy (kJ/mol)

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Absorption of 1 Photon at 3700cm-1, 36.8% of Clusters Excited


One Photon Absorption Causes Two Water Loss

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Energy (kJ/mol)

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Absorption of 1 Photon at 3700cm-1, 36.8% of Clusters Excited


Two Photon Absorption

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Absorption of 2 Photons at 3700cm-1, 18.2% of Clusers Excited

Energy (kJ/mol)

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Another Example of Multiple Water LossDifferent fragment channels of H+(H2O)21

Courtesy of Professor A. Fujii, Tohoku University

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Absorption of 2 Photons at 3700cm-1, 2.9% Absorption


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0.018

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Energy (kJ/mol)

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CsW8 - Absorption of 1 Photon at 3700cm-1


Cs+(H2O)n - Scaled

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Cs+(H2O)n - Not Scaled

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Summary of Small Hydrated Cs+ Spectra

Cs+(H2O)nAr - Not Scaled

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Cs+(H2O)nAr - Scaled

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Multiple Ligand Loss Also Observed in Argonated Clusters

xxxxxxx

quasi-continuum

hvxxxxxx

hvx

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x

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x

nI

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Heat Bath Vibrational Mode Levels

hvxxxxxxx

Energy leaks to

bath

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Repeat

Incoherent absorption: Probability of absorption of n photons proportional to I

Cannot be definitively ruled out.

Model Experiment Provides Much Information

0.0

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Cs+(H2O)n

Total Population at Detector

No Photon 1-4 Photons

Cluster Model Experiment19 27.7% 29.1%18 17.6% 25.2%17 7.0% 5.7%

%Fragmentation

multiple photon absorption in hydrated cesium ion clusters jordan beck, jim lisy june 17,2008 osu...

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