Lasers and Ionization

[Constandinos97]

Hey everyone, I have a question. It might appear stupid to some but I'll go ahead anyway. As you probably know already, ionization is the process when an atom is given enough energy and it loses (or gains) electrons as they get excited to higher states (eventually leaving the atom). And, as we know, it can cause mutations like cancer or leukemia to humans, if the frequency of the wave emitted is below 380nm (approximately). However, I read a YouTube comment where someone claims that non-ionizing lasers (meaning lasers of about 400nm to pretty much anything-infrared or even beyond) can ionize air molecules. Is that true? Do air molecules require less energy to excite electrons? If so, why? I always thought the energy (for example the energy of the ground state) of the states in an atom was the same to every molecule.

 

[joeyss]

You need to have a rapid pulse of a very high powered laser for a very short time.

I think that's the cause. It's beyond anything I ever messed with. All I can say is you better be damn sure you know what you're getting into and have proper eye protection. You'll be blind before you know what the hell hit you if you don't.

 

[Constandinos97]

Eye protection is a given. I'm not talking about that.

I am talking about when a laser has a wavelength shorter than that of UV light. I think cancer is caused by long exposure rather than rapid pulse. And I'm not sure if having a high powered laser is that important.
Since the energy of a photon is depended on the planck constant and the frequency of the wave, I doubt you'll be needing a really strong one to cause the electrons to get ionized. I'm not entirely sure though, If someone knows better please let me know.

 

[Cyparagon]

The method of ionization is direct heating. A flame is ionized gas, too. That doesn't mean candles cause cancer. By the time any of your DNA was ionized (by either a visible laser pulse, or a candle flame), the entire cell would be vaporized as well.

 

[doubleone44]

It is more appropriate to think of air as a mixture of different gases, each one having a different ionization energy. That said, one has to ask oneself which gas they wish to ionize. Let’s look at nitrogen since “air” contains about 78% of it.

Nitrogen has an ionization energy of 1402.3 kJ/mol (amount of energy required to remove 1 mol of electrons from 1 mol of nitrogen gas) or

1402 kJ/mol X 1000 J/kJ / 6.022 X 10^23 = 2.328X10^-18 Joules per atom

Therefore, the wavelength of the laser beam must have at least this much energy to ionize nitrogen. To find the wavelength, we use

E = h*c/lambda

where E is the required ionization energy (in our case 2.328×10^-18), h is Planck’s constant in J*s (Planck constant - Wikipedia, the free encyclopedia), c is the speed of

light (2.98×10^8 m/s) and lambda is the wavelength of the laser beam.

Plugging in these values and solving for lambda, we get a wavelength of approximately 85nm (far ultra-violet light).

What I found using google

source

 

[Constandinos97]

Thanks doubleone44, that's interesting. Unfortunately this doesn't answer my question entirely, but thanks nonetheless

 

[Constandinos97]

I'm sorry Cyparagon, I kinda missed your point. I didn't claim that anything that's ionized can also cause cancer. I merely said that wavelengths beyond ultraviolet may cause ionization and therefore cancer, leukemia etc. So, my question regarding your post, how can a visible laser pulse ionize DNA? I thought ionization requires certain amount of energy which can only be given to electrons through photons of waves of high frequency (such as Ultraviolet, X-Rays and Gamma rays). I know there are different methods of ionization, but I'm talking specifically about this one

 

[NightExplorer96]

^I'd be interesting in knowing as well, I find these kinds of things interesting!

-Alex

 

[Constandinos97]

Me too! I love the physics behind lasers (and physics-especially particle physics-in general). I aspire to become a physicist, but I have 2 and a half more years before going to University (stupid mandatory 2-year old military service ). I can't wait to start learning about the fascinating stuff!

 

[NightExplorer96]

I'm sorry Cyparagon, I kinda missed your point. I didn't claim that anything that's ionized can also cause cancer. I merely said that wavelengths beyond ultraviolet may cause ionization and therefore cancer, leukemia etc. So, my question regarding your post, how can a visible laser pulse ionize DNA? I thought ionization requires certain amount of energy which can only be given to electrons through photons of waves of high frequency (such as Ultraviolet, X-Rays and Gamma rays). I know there are different methods of ionization, but I'm talking specifically about this one

While we wait. I also thought only UV & below could cause mutations in your DNA but if visible light can cause it I assume it's due to how strong it can be if pulsed, causing it to change your DNA.

PS: This is a wild guess coming from a guy who is still in High School so do read this without too much thought

-Alex

 

[Constandinos97]

Haha, no problem, I'm still in High School as well (final year actually) XD Joeyss mentioned something similar. My guess is that, since the energy of a photon depends on the frequency of the wave and the Planck constant, altering the output power wouldn't make a difference. When we are talking about the output power of a laser (I mean its wattage), we are basically talking about the rate at which the photons exit the laser (I don't know if I explained it very well but anyway). So, my guess is that having a really powerful UV laser would simply ionize electrons at a faster rate, not give them more energy. Same applies to visible light. A really powerful 532nm laser, no matter how powerful it is, it can never ionize an electron simply because the energy required depends on its frequency (hence its wavelength) and not its output power. That's what I assume.

Reading your post for a second time, I realized that I might have misunderstood your guess, so let me know XD

 

[NightExplorer96]

Haha, no problem, I'm still in High School as well (final year actually) XD Joeyss mentioned something similar. My guess is that, since the energy of a photon depends on the frequency of the wave and the Planck constant, altering the output power wouldn't make a difference. When we are talking about the output power of a laser (I mean its wattage), we are basically talking about the rate at which the photons exit the laser (I don't know if I explained it very well but anyway). So, my guess is that having a really powerful UV laser would simply ionize electrons at a faster rate, not give them more energy. Same applies to visible light. A really powerful 532nm laser, no matter how powerful it is, it can never ionize an electron simply because the energy required depends on its frequency (hence its wavelength) and not its output power. That's what I assume.

Reading your post for a second time, I realized that I might have misunderstood your guess, so let me know XD

Nope, not at all

It just fascinates me how much there is to learn in this hobby, it's amazing!

-Alex

 

[Cyparagon]

ionization requires certain amount of energy...

Correct so far

...which can only be given to electrons through photons of waves of high frequency

Wrong. There are other ways to ionize.

...I know there are different methods of ionization, but I'm talking specifically about this one

That doesn't make any sense. You've contradicted yourself. "X can ONLY happen with Y. I know it can happen other ways."

As I've said before, The method of ionization in your example is direct heating.

 

[Olwood]

Correct so far



Wrong. There are other ways to ionize.



That doesn't make any sense. You've contradicted yourself. "X can ONLY happen with Y. I know it can happen other ways."

As I've said before, The method of ionization in your example is direct heating.

As suggested in this post there are many mechanisms for ionisation from a laser pulse, and there is still conjecture over which theories are correct. For example proton transfer, which is seen in laser desorption ionisation (LDI), typically generates ions with a 1+ charge. It is an immensely complicated topic!

 

[Constandinos97]

Correct so far



Wrong. There are other ways to ionize.



That doesn't make any sense. You've contradicted yourself. "X can ONLY happen with Y. I know it can happen other ways."

As I've said before, The method of ionization in your example is direct heating.

Yeah, I didn't notice the contradiction when I posted my reply, sorry about that. What I meant is that I was talking particularly about ionization caused by lasers (direct heating using a laser). Wouldn't I be correct then about the fact that you would need waves of greater frequency than that of visible light?

 

[Olwood]

Not necessarily. I prefer to talk in terms of wavelength - shorter wavelength = higher frequency so you're correct to some degree as UV lasers are certainly prevalent in ionisation techniques (see MALDI MS). However visible and even infrared (so longer wavelength, lower frequency) lasers are also used as they favour different mechanisms for the ionisation to occur. Infrared lasers typically deposit a lot of thermal energy, so the dominant processes are thermal.