Molecule Camera Images Hydrogen

Using intensive, ultrashort laser pulses and compiling a film from the separate images, German researchers have "photographed" the smallest and fastest molecule -- hydrogen. The method allows them to visualize the quantum mechanical wave pattern of the vibrating and rotating molecule for the first time.

Because a hydrogen molecule is approximately 5000 times smaller than the wavelength of visible light and vibrates very fast, it is impossible to photograph it using traditional cameras and light microscopes, said the researchers at the Max Planck Institute for Nuclear Physics in Heidelberg. Instead, the researchers have been using pump-probe technology to make high-resolution and ultrahigh-speed images. The molecules are first "bumped" with a "pump" laser pulse and then after a specific time measured with a "probe" laser pulse.

Fig.1: One of the many snapshots that the physicists took of the heavy hydrogen molecule. Each dot in the image represents a specific angle between laser polarization and the molecular axis and a specific distance to the deuterium nuclei. The constellations marked in red occur more frequently. (Graphics: Max Planck Institute for Nuclear Physics)

Because of the hydrogen molecule's size and speed, laser pulses in the past have lasted too long when trying to create an image of the ultrafast molecular motion. The two nuclei in the hydrogen molecule vibrate backwards and forwards so quickly that even visible light only vibrates five times in the same time. Just as in photography, creating a sharp image of fast events requires extremely short exposure times, the researchers said.

To shorten the exposure time, the scientists developed pump-probe apparatus with an average laser pulse duration of only six to seven femtoseconds, allowing molecular motion to be measured continuously for the first time. By comparison, light, which can orbit the earth around eight times in one second, only travels around two thousandths of a millimeter in seven femtoseconds.

To accomplish this, the scientists kept the interval between the laser pulses stable to within 0.3 femtoseconds, during which time light only travels 100 nm. Tthe optical components of the experiment were not allowed to move more than 500 atom diameters in relation to each other while the measurement was being taken.

Deuterium molecules, a compound of two heavy hydrogen atoms, were used for the measurement. They are not energetically excited, and are in the quantum mechanical ground state. The first pump-laser pulse removed an electron from a deuterium molecule and it was ionized. Adjusting to the new situation, the two nuclei of the ionized deuterium molecule moved further apart and vibrated around a new resting position.

The pump pulse also makes the molecule rotate. With the subsequent probe laser pulse the scientists removed the second electron from the molecule; as there are now no more electrons available for fusion and the positively charged nuclei repel each other, the remains of the molecule "exploded"; the closer the two nuclei are to each other when the second ionization takes place, the more violent the explosion.

Using a "reaction microscope" developed earlier, the researchers measure the energy of the two deuterium nuclei, from which they calculate the distance between them and their positions at the moment of explosion. Altering the interval between the pump pulse and the subsequent probe pulse allows a snapshot of the movement of the nucleus at different times to be made (see Fig. 1). A sequence of the separate images produces a "molecular film", giving an insight into the molecular dynamic.

Fig. 2: Development of the wave packet over a period of time. The distance between the deuterium nuclei (R) is plotted against the time. After approximately 100 femtoseconds, the wave packet, i.e. the location of the nuclei, starts to become hazy, after 400 femtoseconds there is a "revival" and the wave packet is put back together again.

In quantum mechanical terms, the vibrating deuterium nuclei are equivalent to a wave packet which starts off as a compact system and after a certain time breaks up -- physicists call this "delocalizing"; it is similar to the way a crowd of differently paced runners initially clumps together at the start of the race and after a while spread out. (This breakup can be seen in Fig. 2.) At the beginning, the movement measured in the wave packet (and thus in the nuclei) is still well localized. After approximately 100 femtoseconds, the structure becomes "fuzzy" or delocalized. The physicists were able to create an image in space and time of this wave packet collapse. They also recorded how the wave packet regrouped and revived after approximately 400 femtoseconds.

Using their extremely fast "molecule camera", the researchers were not only able to create, for the first time, a complete image of the dynamic of one of the fastest molecular systems, but also in a faster time scale than previously achieved. By modelling the pump-laser pulse, they said, the wave packet can created so that certain quantum mechanical processes take place in preference to others. The scientists said they want to use the method to manipulate and control the chemical reactions of larger molecules, and are already conducting experiments on methane molecules in the laboratory in Heidelberg.

The research appears in the Nov. 6 online edition of the journal Physical Review Letters. For more information, visit: www.mpg.de/english/portal/index.html