Introduction - Astronomy



Introduction

The field of chemistry has progressed significantly since the shot-in-the-dark days of alchemy, though it is not entirely different. An understanding of basic physical principles, as well as nearly a millenium of experience has shed much light on the field, but it still involves a lot of educated ‘guess and check.”

It is known that material properties are dependent on energy levels and geometric charge distribution[1], but due to many electron effects, it is not that simple[2]. A great success of physics in this century has been the application of quantum mechanics to describe electron energy state transitions[3]. Currently, in all but the simplest of cases, analytic solutions of material properties are not possible. Theory, however, does make approximations that must be compared to actual data.

An atom or molecule may be excited by a photon of specific energy. The excited atom is unstable, and will lose that energy through a variety of paths. This de-excitation process is referred to as relaxation dynamics. The relaxation dynamics of an atom or molecule play an essential part in photochemical reactions, in addition to serving as a basis on which to evaluate theoretical approximations.

An ion time of flight mass spectrometer was partially assembled to study the relaxation dynamics of molecules following the excitation of K shell electrons by high-energy photons from the Advanced Light Source.

Relaxation Dynamics Background

Photons can interact with molecules in a number of different ways, depending on the photon energy. Photons with sufficient energy (usually in the thousands of electron volts) may excite K shell electrons of an atom. The energy given to the electron is given by the Einstein equation.

E = hν – Φ

hν is the energy of the photon, and Φ is the ionization potential of the electron. A photon with energy equal to the ionization potential may ionize the electron (figure 1).

Decay Paths

The resulting ion has a vacancy in the lower potential 1s orbital, and is in an unstable excited energy state. A loose analogy would be removing the bottom cereal box in a grocery store display. The atom will relax in a number of ways, involving two major paths as indicated in figure 2 [3].

In Radiative transitions (fluorescence) an inner core hole is filled with an electron from a higher shell and a photon is emitted[3]. For Argon, this is written as

Ar + hν ⇒ Ar+(1s-1)+ e-p ⇒ Ar+(3p-1)+ e-p + hν’,

where hν is the frequency of the incident photon, hν’ is the frequency of the emitted photon. The subscript p refers to the fact that the electron was ejected by photoionization. 1s-1 and 3p-1 are shells with missing electrons.

Auger decay occurs when an inner core hole is filled with an electron from a higher shell and an electron is emitted [3]. For Argon, this can be written as

Ar + hν ⇒Ar+(1s-1)+ e-p ⇒ Ar2+(3p-2)+ e-p + e-a,

where the subscripts p and a refer to whether the electron was ejected by photoionization or Auger decay. In this example there are two holes left in the L shell which may further Auger decay, leaving argon with a charge of +4.

A process related to Auger decay is the autoionization of core-hole excited states. [3] This occurs when a core electron is excited to an empty orbital and the excited state decays as follows.

Ar + hν ⇒Ar*(1s-1,4p1) ⇒ Ar+(1s2,3p-1,4p1)+ e-a

Core-hole autoionization may also be referred to as a resonant-Auger process.

Ion Time of Flight Mass Spectrometer

A time of flight mass spectrometer (TOF) can be configured to detect both ions and ejected electrons. A TOF can distinguish between ions by the time it takes them to be accelerated across an electric field. This time is directly proportional to the charge, and inversely proportional to the mass, so the ions are separated into distinct charge-to-mass ratios. By detecting the fragments (ions and electrons) formed after the ionization of gas, the mechanism of ionization can be inferred. The TOF described is the one designed and built by David Hansen, as described in reference 3.

Layout

Figure three shows a cross-section of the TOF, the predominant feature being the parallel plates that regulate the voltage gradient. The light ionizes the gas in the extraction region. The voltage gradient along the TOF is such that the electrons are accelerated toward the electron detector and the ions are accelerated toward the ion detector.

The ion detector is a micro-channel plate (MCP) assembly in which the ion starts an electron cascade with an easily detectable gain of about 106.

Resolution

The resolution of the TOF depends on its ability to reduce the effect of initial spatial and kinetic energy distributions. The effect of initial spatial distributions can be minimized by choosing the spacing and voltages of the plates carefully. This is known as space focusing.

The time it takes for an ion produced in the extraction region to reach the detector is given by

TOF = Textraction+Tacceleration1+Tdrift+Tacceleration2+Tbuffer.

Assuming zero initial kinetic energy and treating the apparatus as a one-dimensional electrostatic situation, it can be shown that [4]

Eextraction = [pic]

Tacceleration1 = [pic]

Tdrift = [pic]

Tacceleration2 = [pic]

Tbuffer = [pic]

where q is the charge of the ion, m is the mass, ΔVn and dn are the width and voltage of the nth region. The distance from the first grid to the location of ion formation is s.

Space focusing takes advantage of the fact that ions formed farther back (a greater s value) gain more energy than those formed closer to the detector. By selecting the right set of voltages and plate spacings, the ion detector can be placed where the ions of differing speeds pass each other. This set of voltages and plate spacings (ΔVn and dn) must be such that [pic] [4].

Increasing the length of ion travel as well as increasing the voltage gradients can minimize the effect of initial velocity of ions.

Signal Processing

The signal from the electron and ion detectors is processed in real time by an array of electronic equipment [3]. A simplified layout is depicted in figure 6.

Constant Fraction Discriminator

The output from the MCP is proportional to the change in electron flow, and is finite in width. The constant fraction discriminator (CFD) converts the signal to a logic pulse to start the time-to-amplitude converter (TAC). Because the amplitude of the MCP signal is variable, some time resolution would be lost if this signal were to start the TAC. The CFD rectifies this situation by providing a timing pulse that is independent of input amplitude. It does this by splitting the signal into two parts. The second signal is delayed and multiplied by some constant fraction and subtracted from the first. A bipolar pulse results, the zero point provides the timing to start the TAC.

Time to Amplitude Converter (TAC)

The TAC provides the time between the detection of the first ion and the second. When the start signal is received, the TAC linearly increases the voltage in a circuit. Upon receipt of the stop signal, this voltage is given to the analog to digital converter, which outputs the corresponding time to the multi-channel analyzer.

Multi-channel Analyzer

The MCA interprets the time as a channel address and increments the count in the appropriate channel by one. The result is an array of channels with different counts that correspond to ions detected. The computer reads the contents of each channel and makes a graph of number of counts as the vertical axis and the time as the horizontal axis. The resulting graph looks like that in figure 7.

Data Collection

The actual situation is a bit different, depending on the mode of data collection. The four possible modes are total ion yield, singles, total coincident ion yield, and photoion-photoion coincidence (PIPICO)[3].

Total ion yield is obtained by scanning the monochromator over an energy range near the ionization energy of core shell electron of the atom under study. The number of ions detected is recorded and plotted as a function of energy. A total coincident ion yield spectra is similar, except it is obtained for a molecule, and the total number of ion pairs at each energy is counted instead of total number of ions.

Singles spectra get their name from the fact that the synchrotron radiation (SR) source must be running in single-bunch (or similar) mode. The SR source produces light by circulating electrons through a storage ring. At each bend, the electrons emit light. In single-bunch mode, the electrons circulate in a bunch, and emit light with a period depending on the physical parameters of the source (656 ns at the ALS). The start signal for the TAC is given by the ion, and the stop signal by the next light burst. The photon energy is kept constant, and the result is a graph of ion counts vs. time as shown in figure 8.

PIPICO spectra are similar, except the species under study is a molecule, and the stop signal for the TAC is provided by the second molecule. Figure 9 is a signal processing setup for PIPICO spectra.

Advanced Light Source

Synchrotron radiation was first observed in the 1940's, when particle physicists were having trouble with energy loss in circular accelerators called synchrotrons. The energy escaped in the form of bright beams of light emanating tangentially to the electron orbit. Experiments with this new type of light first were performed with light from synchrotrons whose primary purpose was particle physics[2].

The Advanced Light Source is the first of a third-generation SR source, designed and operated solely for light emission. The advantages of modern SR are primarily intensity and tunability [2].

Specifications

The ALS produces x-ray light that is 108 times as intense as the brightest x-ray tube. The electrons circulate in the storage ring at the relativistic speed of 99.999994% c with an energy of 1.5 GeV[4]. The value of γ at this speed is approximately 3000 [5]. This intensity greatly reduces the signal/noise ratio, compared to traditional light sources

Lasers can also generate high intensity light, but their wavelengths are limited to a small range. The ALS emits intense radiation in a continuous spectrum from visible to vacuum-ultraviolet light, and from soft x-rays to hard x-rays[2].

Layout

An electron gun injects electrons through a linear accelerator to the booster ring. Once up to speed, they are injected into the storage ring, where they will circulate for a number of hours.

The radiation exits the storage ring via a beamline. Each beamline is set up for different applications, and has a monochromator. The work for this project was conducted at beamline 9.3.1 for the X-ray Atomic and Molecular Spectroscopy (XAMS) group. This beamline was designed specifically for x-ray-spectroscopy applications.

Results

The end result of this project was to get the TOF ready to collect PIPICO data. To this end, a number of small modifications made to the TOF setup. Leaky connections and valves in the gas intake manifold were replaced. An interlock safety mechanism was installed that closed off the chamber from the beamline in an emergency. The signal processing logic was set up as shown is figure 9. A number of minor improvements to the beamline were made, such as replacing some wires with BNC connectors. The TOF was ready to collect data, however the Monochromator for our beamline was inoperable. No data was collected.

References

[1] C. Sparks, Physics Today, May 1981.

[2] B. Crasemann, F. Wuilleumier, Physics Today, June 1984.

[3] D. Hansen, Master's Thesis. University of Nevada, Las Vegas, 1995.

[4] W. Wiley, I. McLaren, Rev. Sci. Instr. (1995).

[5]

[6] D. Attwood, Physics today, August 1992.

Relaxation Dynamics and a Space Focused Ion

Time of Flight Mass Spectrometer

W. Blake Laing

A paper submitted for 1998 Research Experience for Undergraduates.

University of Nevada, Las Vegas

8/7/98

Abstract

Relaxation dynamics of photo-excited atoms and molecules may be studied by the detection of their end products. A space focused ion time of flight mass spectrometer was partially assembled for use at the Advanced Light Source in Berkley, CA.

Acknowledgements

I would like to thank the following people for making this work possible. Dr. John Farley and the University of Nevada Physics department for conducting this program. Dr. Dennis Lindle for the wonderful opportunity to work in Berkley. Dr. David Hansen, Dr. Wayne Stolte, Nathan Allen, and Alex Turnow for their help. Most importantly, Dr. Ray Hefferlin and the physics faculty at Southern Adventist University for sharing their knowledge of, and enthusiasm for, physics.

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3p

3s

2p

2s

1s

Ionization Threshold

hν

Figure 1

Florescence

hν

Ionization Threshold

3p

3s

2p

2s

1s

Auger

Figure 2

Figure 8

Figure 3

electrons

ions

Electron Detector

Light in

Ion Detector

Gas in

Signal processing

electrons

ion

Figure 4: Micro-channel plate

d5

ΔV5=0

Drift tube

Extraction

Acceleration1

Acceleration2

Buffer

d3 ΔV3=0

d4

ΔV4

d2

ΔV2

d1

ΔV1

-5000

0

Figure 6

Figure 5

Second ion

TAC

Stop

start

MCA

CFD

ADC

:

computer

5000

Voltage

Figure 7

fan

gate

stop

e in

Gat

e in

Gat

e in

Gat

k

cloc

or

3

key

2

key

1

key

LT

3

gate

2

gate

1

gate

and

LT

and

LT

i+

320 ns

320 ns

CFD 1 i+

out

out

out

in

CFD 3 i+

out

out

out

in

CFD 2 i+

out

out

out

in

e-

TOF 1

V.Ot

start

stop

TOF 3

V.Ot

start

stop

320 ns

320 ns

TOF 2

V.Ot

start

stop

CFD 1 e-

out

out

in

CFD 3 e-

out

in

CFD 2 e-

out

out

in

BCI

Figure 9

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