Lasers Take 'Snapshots' as Atoms Collide
laser pulses that last just 70 femtoseconds (quadrillionths
of a second), physicists have observed in greater
detail than ever before what happens when atoms
collide. The experiments at JILA, a joint institute
of the National Institute of Standards and Technology
(NIST) and the University of Colorado at Boulder,
confirm a decades-old theory of how atoms—like
tennis balls—briefly lose form and energy
when they hit something. The results will help scientists
study other atomic-scale processes and better understand
the laws of physics.
new data, reported in the Oct. 14 issue of Physical
Review Letters,* provide the equivalent of
missing frames in movies of colliding atoms (see
simulated images in accompanying graphic). As is
the case when a tennis ball is hit by a racquet,
the motion is too quick for the eye but can be detected
using short flashes of light. The JILA scientists
collected data on atoms' properties before, during
and after collisions lasting just half a picosecond
(trillionth of a second) using laser "flashes"
that were even faster.
the JILA experiments, about 10 quintillion potassium
atoms in a dense gas were packed into a titanium
container just 1 square centimeter in size and heated
to 700 degrees C (almost 1,300 degrees F). With
such high temperatures and large numbers of atoms,
the experiment is designed to maximize the number
of atom collisions. Rapidly alternating pulses of
laser light then are used to "freeze frame"
from the first laser pulse is absorbed by the atoms,
placing them in a uniform state, emitting electromagnetic
waves in identical patterns. A second laser then
quickly hits the mass of atoms, and a detector captures
a signal beam formed by the interaction of the beams.
Light from the second pulse is absorbed and re-emitted
by atoms that are "in synch" but not by
atoms that are colliding and losing energy. The
intensity of this signal beam, measured as a function
of the delay between the two pulses, provides a
"snapshot" of how many atoms are colliding
at any one time, as well as details about changes
in their wave patterns.
research was funded in part by the National Science
Lorenz and S.T. Cundiff. 2005. Non-Markovian dynamics
in a dense potassium vapor. Physical Review
Letters. Oct. 14.
'Filling-it-up' Takes More than 'Tank Capacity'
fill up your "empty" fuel tank at the
gas station and the pump charges you for more gallons
than the tank's rated capacity. Are you being deliberately
tampering with pumps does happen, even though state
and local weights and measures officials regularly
check gasoline pumps to ensure their accuracy. But
there are also legitimate reasons for a discrepancy
between the amount of fuel metered by a gas pump
and an automobile's rated fuel tank capacity, according
to a recent paper from the National Institute of
Standards and Technology (NIST).
example, some manufacturers estimate that actual
fuel tank capacity can vary as much as 3 percent
from the tank capacity rating because of design
characteristics, the manufacturing process, and
even the physics associated with the components
that monitor emissions and the electronics of the
fuel system and fuel indicators.
notes that it is important to consider which parts
of a vehicle's fuel tank are used to determine its
capacity rating and what happens to these components
when operating and fueling a vehicle. A small area
at the bottom of a full tank is considered unusable
because the fuel pump cannot reach that level to
draw fuel. In addition, the tank's rated capacity
does not include the “vapor head space,”
the uppermost portion of the tank compartment, nor
does it include the volume of the filler pipe where
fuel enters the vehicle.
Drivers, however, sometimes fill the tank beyond
the pump's automatic shut-off point, resulting in
fuel being drawn into the vehicle's vapor recovery
system or filler pipe. Similarly, if the lanes that
surround the service station pumps are not level,
fuel can shift into the vapor space allowing more
fuel to be delivered into the tank.
cautions against using the “half full”
reading on the fuel gauge to determine the exact
number of gallons it will take to fill the tank.
The fuel gauge is intended as an approximate indication
of the fuel level. Manufacturers may set the “full”
indicator at a level just below the tank's actual
capacity. Reserve fuel also can be present if the
manufacturer designs the fuel gauge to indicate
empty at a level above the actual point where the
tank runs out of gas.
Williams. Fuel Tank Capacity and Gas Pump Accuracy.
August 2005. Weights and Measures Quarterly
newsletter is available at http://ts.nist.gov/ts/htdocs/230/235/archive/B-014.pdf (.pdf;
download Acrobat Reader).
Physicist Honored for Technological Innovation
NIST Fellow Jun Ye was honored Oct. 8 at the "Amazing
Light: Visions for Discovery" symposium held
to celebrate the 90th birthday of 1964 Nobel Laureate
Charles Townes, co-inventor of the laser. Ye won
first prize in the technological innovations category
of the Young Scholars Competition, which honors
promising and innovative physicists under the age
Ye, who leads a research group at JILA, a joint
institute of NIST and the University of Colorado
at Boulder, received the award for his paper "Optical
Phase Control from 10-15 Seconds to
1 Second: Precision Measurement Meets Ultrafast
More than 900 people attended the symposium, held
Oct. 6-8 at the University of California, Berkeley,
with presentations by some 50 top physicists and
researchers, including 16 Nobel Prize laureates.
William D. Phillips, NIST 1997 Nobel Laureate in
physics, was a co-organizer and panelist.
Information about the symposium is available at
Information about the awards is available at www.foundationalquestions.net/townes/Amazing%20Light%20Closing.doc (Microsoft Word document).