ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol. 35           September 5, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. SHUTDOWN SCHEDULE AND TASKS
    3. OZONE PHOTODISSOCIATION PROBED USING UNDULATOR LIGHT
    4. THE OZONE LAYER AND ENVIRONMENTAL CONCERNS
    5. DESIGN CONTEST WINNERS

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam availability for the last two weeks was 92.4% overall and 91.6% during user shifts.

Operations summary for September 5 - September 11
1.9-GeV, 2-bunch operations for users:
    September 5-9, 08:00-23:15
    September 11, 08:00-23:15
1.9/1.5-GeV, 2-bunch operations for users (operating energy to be 
determined based on researchers' needs):
    September 10, 08:00-23:15

2. SHUTDOWN SCHEDULE AND TASKS
(contact: g_krebs@lbl.gov)

The ALS, still a growing facility, will begin a scheduled shutdown for major storage-ring improvements on September 12. The first user beamtime after the shutdown is scheduled for November 1. Following is a list of major tasks to be completed during this shutdown:

    1. The 8-cm-period undulator now in use in sector 9 will be moved to 
sector 12.
    2. A new 10-cm-period undulator will be installed in sector 9, 
allowing chemical dynamics experiments to run at 1.5-GeV storage ring 
energy rather than 1.0 GeV.
    3. The front ends for beamlines 7.3 and 12.0 will be installed.
    4. A new flex-band bellows with windows for diagnostic purposes and 
ports for rf and temperature measurements will be installed in sector 2 
(where a flex band broke earlier this year - see ALSNews Vol. 26, 
May 23, 1995).
    5. The storage ring will be surveyed and several sectors will be 
aligned. This is the length-determining task for the shutdown.

3. OZONE PHOTODISSOCIATION PROBED USING UNDULATOR LIGHT
(contact: arthur@leea.cchem.berkeley.edu)

Scientists from LBNL's Chemical Sciences Division and University of California Santa Barbara have performed a set of experiments at Beamline 9.0.2, using a raw undulator beam for selective ionization of atomic and molecular oxygen (O and O2) formed when ozone (O3) photodissociates. Their experiments have significantly clarified the reaction product dynamics for this process.

In their experiments, a pulsed molecular beam of helium seeded with O3 crosses a pulsed excimer laser beam (193 nm), a high-power source of UV light. The UV light photodissociates the O3 into neutral O and O2, which are then ionized by the undulator beam. A quadrupole mass spectrometer selects products with one charge-to-mass ratio (e.g., O2 as opposed to O), and an ion detector measures their times of arrival in a time-of-flight (TOF) spectrum. This spectrum gives their velocities and, therefore, information about their states of vibrational excitation after photodissociation.

The undulator beam makes a more precise ionizer than electron bombardment (the alternative approach) because it ionizes only the desired photodissociation products, so there is no background noise due to other ionization events. Also, the tightly focused undulator beam causes ionization only in a well-defined location. With electron bombardment, the uncertainty in the ionization location often becomes the factor which limits the resolution of the time-of-flight velocity data.

The distributions of translational kinetic energy determined from the TOF spectra reveal that photodissociation yields a range of vibrational states for the O2 and O products (spectrum). Distinct spectral peaks showing the presence of different product channels had been observed before, but there was no way to determine empirically which peak corresponded to which product channel. The ability to tune the energy of the undulator beam, so that only the molecule or atom of interest is ionized, allowed the researchers to correlate peaks to products unambiguously for the first time.

The experiments at the ALS are an extension of the research team's ongoing investigation into possible new channels for ozone production. It has recently been proposed that if the O2 produced in O3 photodissociation were sufficiently vibrationally excited to react with ground-state O2 to form O3 + O, this sequence of reactions might provide a means for formation of stratospheric O3. The TOF data from the ALS experiments confirmed that there is indeed a substantial yield of suitably excited O2 during photodissociation of ozone at 193 nm. Future work will include a more complete analysis of the results to gain additional insight into the photoionization dynamics of excited states of O2.

The experiments were performed on the crossed-molecular-beam endstation on Branchline 1 of the chemical dynamic beamline (9.0.2). This branchline uses no monochromator, but a newly designed rare-gas filter (see ALSNews Vol. 19, April 4, 1995) suppresses all undulator harmonics above the fundamental. The 8-cm-period undulator (U8) now in use will be replaced by a U10 in September 1995, lowering the beamline's minimum photon energy to 5 eV. Branchline 2 of the beamline obtained first light through its monochromator just last week. The 6.65-meter monochromator will provide users with the highest resolution of any scanning monochromator in the world in its spectral region.

4. THE OZONE LAYER AND ENVIRONMENTAL CONCERNS

If you know the ozone layer is a key environmental issue, but you're not sure why, read on.

Ozone is found in trace quantities throughout the atmosphere, the largest concentrations being located in a layer in the lower stratosphere between the altitudes of 15 and 30 km (9-18 miles). This atmospheric ozone plays a critical role for the biosphere by absorbing ultraviolet radiation (with wavelength between 240 and 320 nanometers), which would otherwise be transmitted to the Earth's surface. This radiation is lethal to simple unicellular organisms (algae, bacteria, protozoa) and to the surface cells of higher plants and animals. It also damages the genetic material of cells (DNA) and is responsible for sunburn in human skin.

Ozone heats the upper atmosphere by absorbing ultraviolet and visible radiation and thermal infrared radiation. This absorption provides enough energy to raise the temperature of the stratosphere (10-50 km or 6-30 miles above the earth's surface) significantly above that of the upper troposphere. (The troposphere is the lowest layer in the atmosphere varying in height from 10- 20 km (6-12 miles) within which nearly all clouds and weather conditions occur.) This increase of temperature with increasing height forms a stable layer resistant to vertical mixing. Consequently, once destructive species such as Cl and ClO are introduced into the ozone layer, it takes roughly 3 years before they can be removed again by mixing down to the ground. This long stratospheric residence time, combined with the catalytic aspect of the ozone destruction caused by these and similar species, makes the ozone particularly sensitive to changes in the composition of the stratosphere.

5. DESIGN CONTEST WINNERS

Thank you to all those who participated in the first (annual, we hope) T-shirt and mug design contest for the ALS Users' Association Meeting on October 23-24. The winners are Eli Rotenberg (University of Oregon, ALS) for the T-shirt and Ernest Fontes (Cornell High Energy Synchrotron Source) for the mug. Their prizes are the chance to judge next year's contest and the satisfaction of knowing their artwork is being worn, used, and displayed on desks around the globe.