To un­der­stand how chem­ical re­ac­tions be­gin, chem­ists have been us­ing super-​slow mo­tion ex­per­i­ments for years to study the very first mo­ments of a re­ac­tion. These days, meas­ure­ments with a res­ol­u­tion of a few dozen at­to­seconds are pos­sible. An at­to­second is 1×10^-18 of a second, i.e. a mil­lionth of a mil­lionth of a mil­lionth of a second.

“In these first few dozen at­to­seconds of a re­ac­tion, you can already ob­serve how elec­trons shift within mo­lecules,” ex­plains Hans Jakob Wörner, Pro­fessor at the Labora­tory of Phys­ical Chem­istry at ETH Zurich. “Later, in the course of about 10,000 at­to­seconds or 10 femto­seconds, chem­ical re­ac­tions res­ult in move­ments of atoms up to and in­clud­ing the break­ing of chem­ical bonds.”

Five years ago, the ETH pro­fessor was one of the first sci­ent­ists to be able to de­tect elec­tron move­ments in mo­lecules on the at­to­second scale. How­ever, up to now such meas­ure­ments could be car­ried out only on mo­lecules in gaseous form be­cause they take place in a high-​vacuum cham­ber.

Delayed trans­port of elec­trons from the li­quid

After build­ing novel meas­ur­ing equip­ment, Wörner and his col­leagues have now suc­ceeded in de­tect­ing such move­ments in li­quids. To this end, the re­search­ers made use of pho­toe­mis­sion in wa­ter: they ir­ra­di­ated wa­ter mo­lecules with light, caus­ing them to emit elec­trons that the sci­ent­ists could then meas­ure. “We chose to use this pro­cess for our in­vest­ig­a­tion be­cause it is pos­sible to start it with high tem­poral pre­ci­sion us­ing laser pulses,” Wörner says.

The new meas­ure­ments also took place in high va­cuum. Wörner and his team in­jec­ted a 25-​micrometre-thin wa­ter mi­cro­jet into the meas­ur­ing cham­ber. This al­lowed them to dis­cover that elec­trons are emit­ted from wa­ter mo­lecules in li­quid form 50–70 at­to­seconds later than from wa­ter mo­lecules in va­pour form. The time dif­fer­ence is due to the fact that the mo­lecules in li­quid form are sur­roun­ded by other wa­ter mo­lecules, which has a meas­ur­able delay ef­fect on in­di­vidual mo­lecules.

Im­port­ant step

“Elec­tron move­ments are the key events in chem­ical re­ac­tions. That’s why it’s so im­port­ant to meas­ure them on a high-​resolution time scale,” Wörner says. “The step from meas­ure­ments in gases to meas­ure­ments in li­quids is of par­tic­u­lar im­port­ance, be­cause most chem­ical re­ac­tions – es­pe­cially the ones that are bio­chem­ic­ally in­ter­est­ing – take place in li­quids.”

Among those, there are nu­mer­ous pro­cesses that, like pho­toe­mis­sion in wa­ter, are also triggered by light ra­di­ation. These in­clude pho­to­syn­thesis in plants, the bio­chem­ical pro­cesses on our ret­ina that en­able us to see, and dam­age to DNA caused by X-​rays or other ion­ising ra­di­ation. With the help of at­to­second meas­ure­ments, sci­ent­ists should gain new in­sights into these pro­cesses in the com­ing years.

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