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“Sounds” of individual molecules captured

Feb. 6, 2008
Courtesy Kansas State University
and World Science staff

Phys­i­cists say they’ve rec­orded ti­ny vibra­t­ions of in­di­vid­ual mol­e­cules, that could be called sounds—de­pend­ing on how you de­fine sound—and put them in au­di­ble form.

The re­sulting bell-like tones can be heard here. But the study went much fur­ther.

The vibra­t­ions in their orig­i­nal form, sci­en­tists said, are too fast and small to hear, but oth­er­wise fit the phys­i­cal de­scrip­tion of what makes a sound: they can pro­duce si­m­i­lar vibra­t­ions in neigh­bor­ing mol­e­cules, which do the same to their neigh­bors, and so forth, spread­ing the os­cilla­t­ions out­ward. 

Part of a graph­i­cal de­pic­tion of the mo­le­cu­lar vi­bra­tions. (Cour­tesy Max Planck Inst. for Nu­clear Phys­ics)


That’s enough to meet some dic­tion­ary def­i­ni­tions of sound, though oth­ers apply the word only to what can be heard. Au­di­ble sound con­sists of the same sorts of vibra­t­ions, but much big­ger and slow­er, and af­fect­ing tril­lions of mol­e­cules, so they can move the ear­drums. 

Uwe Thumm, one of the re­search­ers, said he prefers not to call the ef­fects of a sin­gle mol­e­cule’s vibra­t­ion “sound.” But “that’s a mat­ter of what you de­fine as ‘sound,’” he added. The­re’s no firm line be­tween au­di­ble and inau­di­ble: dif­fer­ent an­i­mals are sen­si­tive to very dif­fer­ent vibra­t­ion char­ac­ter­is­tics, though it’s safe to say none can hear a mol­e­cule.

Mak­ing a mol­e­cule’s vibra­t­ions au­di­ble, how­ev­er, is just a mat­ter of play­ing them back much, much slow­er and louder, re­search­ers said.

To im­ag­ine what one mol­e­cule might sound like if you could hear it, pic­ture the small­est bell you can: it would make a ti­ny, high-pitched ping. Now, try to con­ceive of a tone un­imag­inably smaller and higher. To con­vert this from a fan­ta­sy in­to some­thing you can really hear would re­quire do­ing what the re­search­ers did: replay­ing a re­cord­ing very slowly to low­er the pitch—like a 45-rpm rec­ord be­ing played at 33-rpm—and of course with a vol­ume boost.

To make orig­i­nal the “sound” it­self, Thumm, of Kan­sas State Un­ivers­ity, and col­leagues struck hy­dro­gen atoms with short, in­tense la­ser pulses. They then scaled the vibra­t­ion speeds, or fre­quen­cies, down to about 1,000 Hertz, for a human-au­di­ble pitch.

But they went much fur­ther: they al­so an­a­lyzed the vibra­t­ions al­most as mu­sic, to de­ter­mine how the mol­e­cule, com­posed of mainly of two co­re par­t­i­cles called pro­tons, re­acted to the pulses. “The la­ser pulse ei­ther makes the mol­e­cules vi­brate more vi­o­lently or blows them apart,” Thumm said, which is un­sur­pris­ing be­cause pro­tons are linked by smaller par­t­i­cles, elec­trons, that act as a spring. The pro­tons, banged with a pulse, os­cillate back and forth.

While this may be easy to pic­ture on a large scale, Thumm said par­t­i­cles act dif­fer­ently at the sub­a­tom­ic, or quan­tum lev­el. This means de­ter­min­ing the pro­tons’ loca­t­ions af­ter be­ing hit is­n’t easy. 

It’s si­m­i­lar in a way to what hap­pens if you drop a mar­ble in a bath­tub, he said: look­ing at the cir­cu­lar rip­ples, one can at first tell where the mar­ble was dropped. But when those rip­ples bounce off the tub’s sides, the wave pat­tern changes shape, and it be­comes harder to tell where the mar­ble fell. Thumm said the same thing hap­pens to the pro­tons not with­in sec­onds, but in about 60 bil­lionths of a mil­lionth of a sec­ond. 

Af­ter this, you lose track of the dis­tance be­tween the pro­tons, Thumm said. “All you can say is that they have a cer­tain like­li­hood of be­ing at a cer­tain dis­tance. This is in agree­ment with the bath­tub ex­pe­ri­ment: Sec­onds af­ter the mar­ble was dropped, you can’t tell where ex­actly it plunged in.”

But things work still dif­fer­ently at the quan­tum lev­el. The re­search­ers said they were sur­prised to find that wait­ing about 10 times long­er af­ter the orig­i­nal hit, the proton-proton dis­tance again be­comes de­fi­nite. “We call this a re­viv­al of the orig­i­nal mo­tion,” Thumm said. “It’s not go­ing to hap­pen in the bath­tub, but it hap­pens at the quan­tum lev­el.”

Thumm and col­leagues an­a­lyzed mo­lec­u­lar mo­tions by break­ing them in­to their var­i­ous fre­quen­cies, or vibra­t­ion speeds. Dif­fer­ent fre­quen­cies are what cre­ate dif­fer­ent pitches in mu­sic. The mo­lec­u­lar fre­quen­cies could thus be an­a­lyzed as if they were mu­sical chords.

This stu­dy—based on ex­pe­ri­ments at the Max Planck In­sti­tute for Nu­clear Phys­ics in Hei­del­berg, Ger­many—bore out the pic­ture of “re­viv­al” of mo­tion, Thumm said. And hap­pi­ly, he added, this al­so con­firmed pre­dic­tions he pub­lished in 2003 with the in­sti­tute’s Bernold Feuer­stein. The agree­ment “was al­most per­fect and ex­ceeded our ex­pecta­t­ions,” Thumm said.

One can lit­er­ally “lis­ten to the vibra­t­ions and hear the ‘re­viv­al,’” Thumm said.

Thumm said re­search­ers hope to be able to do the same thing for more com­plex mol­e­cules like wa­ter or meth­ane. Just as a C Ma­jor chord sounds dif­fer­ent from a d mi­nor chord, Thumm said oth­er mol­e­cules al­so would have their own un­ique sound. The stu­dy, pub­lished in the Oct. 10 is­sue of the re­search jour­nal Phys­i­cal Re­view Let­ters, could al­so help in­ves­ti­ga­tors in the goal of ap­ply­ing la­sers to steer chem­i­cal re­ac­tions, Thumm said.


* * *

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Homepage image: In a plot of frequency vs. distance, the light blue strips represent the likely distances separating the two oscillating hydrogen atoms at different frequencies in the experiment.



 

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Physicists say they’ve recorded tiny vibrations of individual molecules, which could be called sounds—depending on how you define sound—and put them in an audible form, as bell-like tones. The results can be heard here. But the study went much further. The vibrations in their original form, scientists said, are too fast and small to hear, but otherwise fit the physical description of what makes a sound: they can produce similar vibrations in neighboring molecules, which do the same to their neighbors, and so forth, spreading the oscillations outward. That’s enough to meet some dictionary definitions of sound, though others apply the word only to what can be heard. Audible sound consists of the same sorts of vibrations, but much bigger and slower, and affecting trillions of molecules, so they can move the eardrums. Uwe Thumm, one of the researchers, said he prefers not to call the effects of a single molecule’s vibration “sound.” But he added, “that’s a matter of what you define as ‘sound.’” There’s no firm line between audible and inaudible: different animals are sensitive to very different vibration characteristics, though it’s safe to say none can hear a molecule. Making a molecule’s vibrations audible, however, is just a matter of playing them much, much slower and louder, researchers said. To imagine what one molecule might really sound like if you could hear it, picture the smallest bell you can: it would make a tiny, high-pitched ping. Now, try to conceive of a tone unimaginably smaller and higher. To convert this from a fantasy into something you can really hear would require doing what the researchers did: replaying a recording very slowly to lower the pitch—like a 45-rpm record being played at 33-rpm—and of course with a volume boost. To make original the “sound” itself, Thumm, of Kansas State University, and colleagues struck hydrogen atoms with short, intense laser pulses. They then scaled the vibration speeds, or frequencies, down to about 1,000 Hertz, for a human-audible pitch. But they went much further: they also analyzed the vibrations almost as music, to determine how the molecule, composed of mainly of two core particles called protons, reacted to the pulses. “The laser pulse either makes the molecules vibrate more violently or blows them apart,” Thumm said, which is unsurprising because protons are linked by smaller particles, electrons, that act as a spring. The protons, banged with a pulse, oscillate back and forth. While this may be easy to picture on a large scale, Thumm said particles act differently at the subatomic, or quantum level. This means determining the moving protons’ locations after being hit isn’t easy. It’s similar in a way to what happens if you drop a marble in a bathtub, he said: looking at the circular ripples, one can at first tell where the marble was dropped. But when those ripples bounce off the tub’s sides, the wave pattern changes shape, and it becomes harder to tell where the marble fell. Thumm said the same thing happens to the protons not within seconds, but in about 60 billionths of a millionth of a second. After this, you lose track of the distance between the protons, Thumm said. “All you can say is that they have a certain likelihood of being at a certain distance. This is in agreement with the bathtub experiment: Seconds after the marble was dropped, you can’t tell where exactly it plunged in.” But things work still differently at the quantum level. The researchers said they were surprised to find that waiting about 10 times longer after the original hit, the proton-proton distance again becomes clear. “We call this a revival of the original motion,” Thumm said. “It’s not going to happen in the bathtub, but it happens at the quantum level.” Thumm and colleagues analyzed molecular motions by breaking them into their various frequencies, or vibration speeds. Different frequencies are what create different notes in music. The molecular frequencies could thus be analyzed as if they were musical chords. This study—based on experiments at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany—bore out the picture of “revival” of motion, Thumm said. And happily, he added, this also confirmed predictions he published in 2003 with the institute’s Bernold Feuerstein. The agreement “was almost perfect and exceeded our expectations,” Thumm said. One can literally “listen to the vibrations and hear the ‘revival,’” Thumm said. Thumm said researchers hope to be able to do the same thing for more complex molecules like water or methane. Just as a C Major chord sounds different from a d minor chord, Thumm said other molecules also would have their own unique sound. The study, published in the Oct. 10 issue of the research journal Physical Review Letters, could also help investigators in the goal of applying lasers to steer chemical reactions, Thumm said.