Crossed molecular beam experiments are chemical experiments where two beams of atoms or molecules are collided together to elucidate the dynamics of the chemical reaction, and can detect individual reactive collisions.

Technique

In a crossed molecular beam apparatus, two collimated beams of gas-phase atoms or molecules, each dilute enough to ignore collisions within each beam, intersect in a vacuum chamber. The direction and velocity

Velocity

In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 of the resulting product molecules are then measured, and are frequently coupled with mass spectrometric

Mass spectrometry

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 data. These data yield information about the partitioning of energy among translational

Translation (physics)

In physics, translation is movement that changes the position of an object, as opposed to rotation. For example, according to Whittaker:...

, rotational

Angular momentum

In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

, and vibration

Vibration

Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...

al modes

Normal mode

A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies...

 of the product molecules.

History

The crossed molecular beam technique was developed by Dudley Herschbach and Yuan T. Lee

Yuan T. Lee

Yuan Tseh Lee, Ph.D. is a chemist. He was the first Taiwanese Nobel Prize laureate, who, along with the Hungarian-Canadian John C. Polanyi and American Dudley R. Herschbach won the Nobel Prize in Chemistry in 1986 "for their contributions to the dynamics of chemical elementary processes"...

, for which they were awarded the 1986 Nobel Prize in Chemistry

Nobel Prize in Chemistry

The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature,...

. While the technique was demonstrated in 1953 by Taylor and Datz

Sheldon Datz

Sheldon Datz was born in New York City, son of Clara and Jacob Datz. He went to Stuyvesant High School and received degrees in Chemistry from Columbia University and University of Tennessee. He did early work inventing the molecular beam technique with Dr. Ellison Taylor which later won the...

 of Oak Ridge National Laboratory

Oak Ridge National Laboratory

Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...

, Herschbach and Lee refined the apparatus and began probing gas-phase reactions in unprecedented detail.

Early crossed beam experiments investigated alkali metals such as potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

, rubidium

Rubidium

Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...

, and cesium. When the scattered alkali metal atoms collided with a hot metal filament, they ionized, creating a small electric current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

. Because this detection method is nearly perfectly efficient, the technique was quite sensitive. Unfortunately, this simple detection system only detects alkali metals. New techniques for detection were needed to analyze main group element

Main group element

In chemistry and atomic physics, main group elements are elements in groups whose lightest members are represented by helium, lithium,...

s.

Detecting scattered particles through a metal filament gave a good indication of angular distribution

Probability distribution

In probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....

 but has no sensitivity to kinetic energy. In order to gain insight into the kinetic energy distribution, early crossed molecular beam apparatuses used a pair of slotted disks placed between the collision center and the detector. By controlling the rotation speed of the disks, only particles with a certain known velocity could pass through and be detected. With information about the velocity, angular distribution, and identity of the scattered species, useful information about the dynamics of the system can be derived.

Later improvements included the use of quadrupole mass filters

Quadrupole mass analyzer

The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...

 to select only the products of interest, as well as time-of-flight mass spectrometers

Time-of-flight mass spectrometry

Time-of-flight mass spectrometry is a method of mass spectrometry in which an ion's mass-to-charge ratio is determined via a time measurement. Ions are accelerated by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has...

 to allow easy measurement of kinetic energy. These improvements also allowed the detection of a vast array of compounds, marking the advent of the "universal" crossed molecular beam apparatus.

The inclusion of supersonic nozzles

De Laval nozzle

A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...

 to collimate the gases expanded the variety and scope of experiments, and the use of laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

s to excite the beams (either before impact or at the point of reaction) further broadened the applicability of this technique.