Question

Problem 4 – Fun with Vacuum Physics

To do surface physics, we need to put the entire experiment into an ultra high vacuum so the air does not contaminate the surface. To heat the metal sample, we use an electron gun. A 500 V power supply has the positive lead connected to the metal sample, and the negative connected to a filament that emits electrons. The electrons boil off of the filament at 0 V, and are attracted to the sample at 500 V.

a) What is the kinetic energy in Joules of one electron when it hits the sample?

K = _______________ (This is called 500 eV of energy)

b) How many electrons per second must hit the sample to provide 1 W of heating power?

c)To prepare the sample for the experiment, we need to start by removing the top 5 atomic layers from the sample, since they have been contaminated with molecules from the trace amount of gas in the vacuum. To do this, we ionize helium atoms and then accelerate them using the same device as the electron gun, but switching the positive and negative leads, since the ion is 4He+. Find the kinetic energy, momentum, and velocity of a singly charged helium atom when it has been accelerated through 500 V

K = _______________ p = _______________ v = _______________

d) Since the helium ion is so much bigger than the electron, instead of getting absorbed into the metal and heating it, it collides with the surface, and about 40% of the time it knocks off one atom from the surface. Assuming that the surface of the sample is 2 cm2 in area, the atoms are arranged in a square pattern with a distance of m between nearest neighbors, and it takes 6 minutes to remove the top 5 atomic layers, how many ions per second must be hitting the sample?

= _______________

While this may seem like a big number, it is tiny compared to Avagadro's number, which is important since we will need to pump out of our vacuum chamber every atom of gas that we add!

Thank you.

Answer 1